Important terms used in GIS

ABSOLUTE ACCURACY:
It is a measure of the difference between the location of the object as specified in a GIS and its true location in the real world

ADAPTIVE SAMPLING:
It is a data sampling technique that uses accumulated knowledge from samples already taken to direct future sampling. Ex: Redundant sample points may be rejected during the sampling process on the grounds that they carry too leittle extra information

ADDRESS GEOCODING:
It refers to the process of alphanumeric locational identifiers to spatially related information. The process implies a geographic base file which can be used to pass addresses in order to determine characteristics about the geometry.

ALBEDO:
The ratio of light reflected by a planet or satellite to that received by it.

ALIASING:
The appearance of jagged lines on a raster display

ANTI-ALIASING:
Anti-aliasing removes or greatly smoothes the jagged, stair-step appearance of a digital line by filling-in some of the intermediate and flanking cells in lower-intensity colors.

Arc:
A portion of the perimeter of a two dimensional closed figure lying between two nodes at which two or more arcs intersect. An arc usually represents a common boundary between two adjoining mapping units.

ARC SECOND:
The SIXTIETH part of of a minute of angular measuere represented by "

AREA:
A level of spatial measurement referring to a two dimensional defined space.

ASPECT:
It refers to the compass direction (usually from North) of the line of steepest slope at some selected point.

ASPECT RATIO:
It refers to the ratio of horizontal scale to vertical scale for printing or display

ATTRIBUTE:
It refers to the set or collection of data that describe the characteristics of real world entities or conditions

ATTRIBUTE QUERY:
It is the process of selecting data items from a file based system, based on values of specific attributes or combinations thereof defined by arithmetic, relational and logical expressions

AUDIT TABLE:
In a GIS, a table of information describing a maps subjects, items, perimeters and areas.

AUTOCORRELATION:
It refers to the statistical concepts expressing the degree to which one value of an attribute covaries with other values of the same attribute. Especially in the spatial case, it refers to the degree to which the values of an attribute of two objects covary with the distance separating them.
Mathematical autocorrelation techniques can be applied to overlapping image segments in processes such as mosaicking and raster-to-vector registration. Ex: Autocorrelation can automatically find the best seam between overlapping image segments.

AUTOMATED CARTOGRAPHY:
It is the process of drawing maps using computer driven display devices such as plotters and graphic screens.

AVHRR IMAGERY:
Advanced Very High Resolution Radiometer Imagery produced by NOAA satellites

AVIRIS IMAGERY:
Airborne Visible Infra-Red Imaging Spectrometer imagery. These are multispectral images of approximately 240 coregistered spectral bands collected by NASA aircraft.

AZIMUTH:
The angle defined by the intersection of a map's central line of projection with any meridian. If a map projection uses a central line that is oriented to the TRUE NORTH, such as a standard meridian, the azimuth is 'ZERO'.

AZIMUTHAL PROJECTION:
It is a class of map projections in which the directions of all lines radiating from a central point are the same as the directions of corresponding lines on the sphere. Azimuthal projections are formed onto a plane which is usually tangent to the globe at a pole (polar projection), at a point on the equator or any selected intermediate point. Most azimuthat maps do not have standard parallels or standard meridians. Each map has only one standard point, "THE CENTER".
*Azimuthals are suitable for minimizing distortion in a circular region such as antarctica BUT NOT for an area with a predominant length in on direction*

BAND OR SPECTRAL BAND:
A range of wavelength of electromagnetic radiation.

BASE DATA:
It is the base level of map data on which other information is placed for purposed of comparison or graphical correlation. Map data that RARELY CHANGES and is USED  REPEATEDLY is called base data.

BEARING:
It is the horizontal angle of a direction, measured in the quadrant of the line as degrees East or West.
Ex: NE = 45 degrees East of North; SW = 45 degrees West of South

CADASTRAL LAYER:
A set of information depicting the pattern of land ownership rights in an area.

CARDINAL DIRECTION:
The four principal directions: North, East, West and South.

CARTESIAN COORDINATES:
A coordinate system in which location of points in space is expressed by reference to three perpendicular axes (x,y,z)

CARTOGRAPHY:
The art or science of making maps

CELL:
One value in a raster that corresponds to a specific area on the ground

CENTRAL MERIDIAN:
The North-South meridian of a map projection around which the map is centered.

CHANGE IMAGE:
An image produced using raster algebra that shows change over time between coregistered images. (Multitemporal image processing)
Ex: Subtracting old raster image from New raster object could show the difference between early season crop development and mid-season development or between pond surface area from year to year.

CHOROPLETH MAP:
A map with areas coloured or shaded such that the darkness or lighness of an area symbol is proportional to the density of the mapped phenomenon. It is a map of uniform values separated by abrupt boundaries. ADJACENT AREAS ARE NOT NECESSARILY CLOSE IN VALUE.

CIR IMAGE:
Colour Infra Red Image. These images are collected by an electron scanner or a camera that uses a special film with sensitivity from green through infrared. Photographic infrared radiation just beyond the range of human vision is displayed as red. Normal red from the scene becomes green and green becomes blue. Normal blue is filtered out and not recorded.
CIR images are used to show vigor of plant life. Healthy vegetation appears red while distressed or damaged vegetation appears pink, tan or yellow.

CLUMP:
It represents a set of contiguous line, node and polygon elements in a vector object

CLUSTERING:
It is a process in which multiple, spatially coincident, coregistered raster objects are reduced to a single raste object called a cluster map.

CMY:
Cyan-Magenta-Yellow

COGO:
Coordinate Geometry. A set of mathematical tools and functions for encoding and converting bearings, distances, angles etc into coordinate information. Data is input and the geometry is determined automatically.

COMPLEX CORRELATION:
It is the ability to compare maps representing different time periods, extracting time differences, or computing indices of change. It is a multitemporal analysis function.

COMPLEX GENERALIZATION:
It refers to generalization that may require change in the type of an object or relocation in response to cartographic rules.

COMPOSITE MAPS:
It refers to a single map created by joining together several separately digitized or scanned maps.

COMPRESSION:
It is a method for reducing the file size usually using a run-length coding algorithm

CONDITIONAL MAP ELEMENT:
It refers to a piece of map upon which society places conditions. Ex: Land use, Zoning, Historic district etc.

CONNECTIVITY ANALYSIS:
It is the ability to identify areas or points that are not connected to other areas or points by linear features.

CONTIGUITY ANALYSIS:
It refers to the adjacency relationships between any given polygon and its neighbours. This involves summarizing and relating attributes of neighbouring polygons to the polygon being examined.

CONTINUOUS DATA:
Data in a raster object is said to be continuous if it can be represented by a three dimensional surface such that intermediate values can be derived with meaningful results.

CONTOUR (N):
It is an imaginary line on the ground, all points of which are at the same elevation above or below a specific datum (M.S.L.)

CONTOUR (V):
It refers to interpolation of elevation of points at specific intervals when elevations of a set of regularly or irregularly spaced points is given.

CONTOUR MAP:
It is a topographic map that uses contour lines to portray relief . Contour lines join points of equal elevation.

CONTROL POINT:
Points and or cells which are used to establish map coordinate control for uncalibrated objects. In the manual mosaic process, a control point is a feature in a piece of the mosaic for which the map coordinates are known. In the raster-to-vector calibration process, a control point is a feature that is co-located between the uncalibrated raster object and the calibrated vector overlay. A control point shows on both a raster object and an overlaying vector object.

Salient features of selected projections

SALIENT FEATURES OF POLYCONIC PROJECTION

1. All parallels are projected without distortion (Scale is exact along all parallels)
2. Parallels are arcs of circles but they are not concentric
3. It is neither conformal nor equal area
4. Central meridian and equator are straight lines; All other meridians are complex curves
5. There is NO DISTORTION ONLY AT CENTRAL MERIDIAN
6. It is used in India for all topographical mapping on 1:25,000; 1:50,000 and 1:250,000 scales

The disadvantages of this projection are listed below:

1. It can have a rolling fit only
2. Meridians and parallels do not intersect at right angles
3. Inability to show seamless data in a rectangular coordinate system

The polyconic projection is not being used for mapping anywhere in the world except India and a few adjacent countries.

SALIENT FEATURES OF LAMBERT CONFORMAL CONIC PROJECTION

1. It is a conical projection
2. It is conformal
3. Parallels are unequally spaced arcs of concentric circles, more closely spaced near the center of the map
4. Meridians are equally spaced radii of the same circles, thereby cutting parallels at right angles
5. Scale is true just along two standard parallels or along just one
6. Intersection of central parallel and central meridian is the origin of rectangular coordinate system
7. Central meridian is Y-axis and a line perpendicular to it is X-axis
8. A large value is given to the origin so that ALL coordinates for the projection are POSITIVE
9. The origin assumes a value (0,0) is also known as 'FALSE ORIGIN'

SALIENT FEATURES OF TRANSVERSE MERCATOR PROJECTION

1. It is a widely used conformal projection
2. It is a cylindrical projection
3. It is conformal
4. The central meridian, each meridian 90 degrees from the central meridian and equator are straight lines
5. Other meridians and parallels are complex curves
6. Scale is true along central meridian or along two straight lines equidistant and parallel to central meridian
7. Scale becomes infinite 90 degrees from central meridian
8. It is used extensively for quadrangle maps at scales 1:24,000 to 1:250,000
9. It was presented by lambert in1772
10. It is not used in India but extensively used in USA
11. The State Plane Coordinate System (SPCS) is based on the Transverse Mercator Projection in USA for states with North-South extent
12. It is used for quadrangle maps in the USA
13. It is used for army map service in the USA
14. Ordnance Survey of Great Britain switched to transverse mercator from cassini
15. It is used in Canada in three zones.

SALIENT FEATURES OF UNIVERSAL TRANSVERSE MERCATOR PROJECTION

1. It is a particular case of transverse mercator projection
2. Transverse mercator in this projection is 6 degrees wide
3. Reference ellipsoid for North America was given by Clark in 1866
4. Central meridian is the origin for the longitude
5. Equator is the origin for the latitude
6. Unit for distance is metre (m)
7. False northing is 0m for northern hemisphere and 10,000,000m for southern hemisphere
8. False easting is 500,000m
9. Scale factor at central meridian is 0.9996
10. Zone numbering begins at 1 for zone between 180 W and 174 W and increases to 60 for zone bounded by meridians 174 E and 180 E (Each zone 6 degrees wide)
11. Latitude limits 80 N and 80 S
12. Nearly 60 countries use this projection as the general use projection within the country
13. It is NOT the universal projection for all countries.
14. In India, it is used by NAVAL HYDROGRAPHIC SURVEYS for their maps and charts

Conversion among coordinate systems are carried-out mathematically using map projection equations and their inverses.

Characteristics of maps and map projections

Characteristics of maps:

1. Maps are always concerned with TWO elements of reality:
1. Location (spatial data) and 
2. Attributes (aspatial or non-spatial data)
2. Maps are reductions of true surfaces. They are two dimensional representations of the earths surface drawn to scale.
3. Maps are usually outdated representations
4. Maps are ALWAYS STATIC VERSIONS
5. Maps cannot be updated. Updation requires preparation of a new map.

A map is a traditional method for storing, analysing and presenting spatial data

Topology is based on the geometric relationship of objects in a map

The purpose of a map is to turn data into information that will be communicated to the user.

Scale is defined as the ratio of a distance on the map to the corresponding distance on the ground. The units for distance should be the same.

THREE BASIC SYMBOL TYPES used are point, line and areas. They are used to represent real world features. The method used to represent a spatial feature DEPENDS ON THE SCALE USED. 

The relationship between scale and detail is called SCALE RELATED GENERALIZATION.

PROJECTION

-The Earth appears flat at close range
-The Earth is roughly SPHERICAL (as displayed in satellite images)
-Cartographers developed a set of techniques called "MAP PROJECTIONS" to depict the spherical earth in two dimensions with reasonable accuracy.

Imagine a football (inflated) with the image of the Earth on it = 3D representation of Earth
Now deflate the football = 2D representation of Earth

Projection is the process of placing a light bulb in transparent globe on which OPAQUE Earth features are placed and projecting the feature outlines on a 2D surdace surrounding the globe. The globe could be projected on:
-a flat piece of paper
-surrounding the globe in a cylindrical fashion or
-surrounding the globe in a cone

Each of the above three projections form a projection family called:
-Planar projection
-Cylindrical projection
-Conical projection and
-Azimuthal projection

-Projections are not absolute accurate representations of geographical space. The characteristic of maps that must be retained for accurate analytical operations dictate which projections must be used.

-IT IS IMPOSSIBLE TO PRESERVE ALL PROPERTIES AT THE SAME TIME WHEN PERFORMING A MAP PROJECTION.

When performing a map projection, selection of a map projection will be based on what property needs to be preserved. The properties that need to be considered are:
-angles
-shapes
-distances
-directions and
-areal sizes

Angular conformity / Conformal / Orthomorphic projection MAINTAINS correct angular correspondence. This leads to:
-Distortion of areas
-Incorrect measurement

-Equal area / Equivalent projections preserve areas

-Equidistant projections preserve distances

There is NO IDEAL MAP PROJECTION

A map projection can be defined as representation of meridians and parallels portraying the curved surface of the datum surface on a two dimensional plane.

The two surfaces should have a one to one correspondence with each other.

Origin is generally chosen as the intersection of CENTRAL MERIDIAN with CENTRAL PARALLEL.

1. Map projections transfer the spherical earth onto a two dimensional surface thereby approximating the true shape of the earth. This introduces errors into spatial data.
2. A projection is a method by which the curved surface of the earth is represented on a flat surface by using mathematical transformations between location of places on earth and their projected locations on the plane.
3. When curved surface of the earth is shown on a plane, DISTORTION IS INEVITABLE
4. Distortion is LEAST when the map shows SMALL AREAS and MAXIMUM when the map shows ENTIRE SURFACE OF THE EARTH.

Projections are broadly classified into:

1. EQUAL AREA PROJECTIONS (display correct area)
2. CONFORMAL PROJECTIONS (display correct shape/directions) and
3. EQUIDISTANT PROJECTIONS (display correct distance)

Equal Area Projections (EAP) are used for estimating resource, forest coverage, etc in a region.

Conformal Projections (CP) are used in navigation purposes requiring accurate directions.

CP are also called orthomorphic projections

Ex: Lambert Conformal Conic and Transverse Mercator

Universal Transverse Mercator (UTM) is a special case of transverse mercator

Examples of Non-Conformal Projections are:

1. Polyconic
2. Cassini
3. Alberts Equal area, etc

Important projections for mapping in India:

1. Polyconic projection
2. Lambert Conformal Conic projection
3. Transverse mercator projection
4. Universal Transverse Mercator projection and
5. Cassini projection

All geographic surfaces are in TWO tangible formats:

-Discrete: They occupy a given point in space and time (Trees, Houses, etc)

They have zero dimensionality BUT some spatial dimension

-Continuous: They possess infinite number of possible height values distributed without interruption across the surface (Cliff,trenches, ridges, hills, etc)

They are described by:

-Citing their locations

-The area occupied by the feature and

-Their orientation with the addition of the third dimension

-Topographic map shows BOTH DISCRETE and CONTINUOUS information

-Elevation is shown as a series of contour lines

-Man-made features are shown by a lines and shapes

-Different kinds of information that is stored in various ways is called THEME

-DATUM PLANE is the reference surface from which all altitudes are measured. Usually, Datum plane = Mean Sea Level (MSL)

-ELEVATION or ALTITUDE is the vertical distance between GIVEN POINT and DATUM PLANE.

-Height is defined as the vertical difference between an object and its surroundings.

-Difference in elevation of an area between tops of hills and bottoms of valleys is known as relief of the terrain.

-A point of known elevation and position is indicated on a map by the letters B.M (Bench Mark) with the altitude given to the nearest foot.

-A map line connecting points representing places on the Earth's surface that have the same elevation is called CONTOUR LINE.

-Contours represent the THIRD DIMENSION on a map

-The difference in elevation represented by adjacent contours is called CONTOUR INTERVAL.

-Maps are an important for of input to a GIS and a common means to portray the results of analysis from a GIS

-The TWO FUNDAMENTAL ASPECTS OF REALITY that maps and GIS are connected with:

-LOCATIONS and

-ATTRIBUTES

using these, several TOPOLOGICAL and METRIC properties of a relationship can be defined. Eg:

-Distance

-Direction

-Connectivity

-Proximity, etc

SYMBOLOGY:

-Artificial works shown in BLACK

-Water features (streams, swamps and glaciers) shown in BLUE

-Relief shown by contours in BROWN

-Major highways shown in RED

-Areas of woods, orchards, vineyards and scrub shown in GREEN.

-SRS SPATIAL REFERENCING SYSTEM

-GCS GEOGRAPHIC COORDINATE SYSTEM

-RCS RECTANGULAR COORDINATE SYSTEM

-NCS NON-COORDINATE SYSTEM

-SGS SPHERICAL GRID SYSTEM

-CRS COORDINATE REFERENCING SYSTEM

Longitudes = Meridians (Drawn pole to pole)

Longitudes start at GREENWICH (England) also called PRIME MERIDIAN 

(Numbered East to West)

Corresponding meridian on the opposite side of the globe is called: 

INTERNATIONAL DATE LINE

Latitudes lie at RIGHT ANGLES to lines of longitudes and run parallel to each other.

Latitudes = Parallels

Transformation of 3D space to 2D map distorts atleast one of the following:

-SHAPE

-AREA

-DISTANCE or

-DIRECTION

Angular conformity / Conformal / Orthomorphic projections MAINTAIN correct angular correspondence

This leads to:

-Distortion of areas

-Incorrect measurement

Equal area / Equidistant projections PRESERVE AREAS

Equidistant projections PRESERVE DISTANCES

The three projections mentioned above are MUTUALLY EXCLUSIVE

THERE IS NO IDEAL MAP PROJECTION

To transfer the image of the Earth with its irregularities on to a plane surface of a map, THREE factors are involved are:

-GEOID

-ELLIPSOID or ELLIPSOID WITH DATUM

-PROJECTION

The geographical relationships of the Earth in three dimensional form is transferred into two dimensional plane of a map by a process known as "MAP PROJECTION"

-ALL projections developed, fall into one of the following categories:

-CONIC PROJECTION

-CYLINDRICAL PROJECTION and

-PLANAR PROJECTION

-Every flat map MISREPRESENTS the Earth in some way. NO MAP CAN TRULY REPRESENT THE SURFACE OF THE ENTIRE EARTH.

A map or parts of a map can show one or more, but NEVER ALL of the following:

-True shapes

-True direction

-True distance

-True areas

A combination of any two of the above projections forms a hybrid projection

-CONIC PROJECTIONS are suited to map areas having EAST-WEST extent. Ex: USA, Canada, Peoples Republic of China

If a sheet of paper is laid tangentential to a point on the globe and the geographical features of the globe are transferred on it, the projection obtained is called, AZIMUTHAL PROJECTION. In this projection, straight lines intersect the designated center point and parallels appear as concentric circles around the center point.

ADLER has named FIVE BASIC CRITERIA for CLASSIFICATION OF MAP PROJECTIONS.

-Nature of projection surface as defined by geometry

-Contact of projection surface with DATUM surface

-Alignment of projection surface with relation to the datum surface

-Cartographic requirements and

-Mode of generation of DATUM SURFACE and coordinate system

TO MAINTAIN ACCURACY:

-The Earth is a SPHERE for small scale maps and

-The Earth is a SPHEROID for large scale maps

In an ellipsoid or sphere, the latitude and longitude are mentioned in degrees, minutes and seconds of arc.

The plane system of rectangular X and Y coordinates is referred to as EASTING and NORTHING respectively

Commonly used map projections are:

-MERCATOR

-TRANSVERSE MERCATOR

-OBLIQUE MERCATOR

-POLYCONIC PROJECTION

-LAMBERT CONICAL ORTHOMORPHIC PROJECTION

-GRID SYETEMS

-LAMBERT GRID SYSTEM FOR INDIA

-UNIVERSAL TRANSVERSE MERCATOR (UTM) GRID

Uses and limitations of paper maps

USES OF MAPS:

1. Maps have been used since time immemorial for navigation and military purposes.
2. Maps are used to organise geographic data. 
1. Ex: Topography, 
2. Natural resources (thematic maps contain information about a specific theme - geology, soils, roads, ecology, hydrology, etc)
3. Political (Abstract boundaries for public, private, national and international levels)
4. Information types - Qualitative (Ex: land use classes) and Quantitative (Ex: depth to bedrock)
5. Map types - Choropleth (areas of equal area separated by boundaries (landuse)) and isolines (or contours)

LIMITATIONS OF PAPER MAPS:

1. Maps have to be genaralised to make them readable. Important details may be lost for site specific analysis.
2. Small scale maps representing large areas must be represented on a large number of map sheets making viewing and analysis difficult
3. Data retrieval is difficult
4. Printed maps are ""static
5. Combining different thematic maps for land suitability or spatial analysis is very difficult
6. Map updation is a tedious process
7. New technologies for gathering information are better accommodated in digital systems
8. Complexity of urban and natural resource problems increases the need for sophisticated analysis techniques.
9. With the availability of low cost digital computers and greater access to data, the shortcomings of paper maps can be overcome easily using a digital GIS.

Maps

"A map is a set of points, lines and areas that are defined by their spatial location with respect to a coordinate system and by their non-spatial (aspatial) attributes." A map legend links the non-spatial attributes to the spatial attributes.
A map is an abstraction of the real world
A map is a graphical representation at a certain level of detail which is determined by the scale. A map is a graphical representation of location of geographic features both 'explicitly' and 'relative' to one another

Map sheets have physical boundaries and features spanning two maps have to be cut into pieces.

Cartography is the art and science of map making.

A map is a 'model' of the real world

Each feature is defined by:

- its location in space (with reference to a coordinate system) and

-its characteristics (attributes)

A very important factor in the production of maps is scale or representative fraction (R.F). Map scale is a ratio of the distance on the map to the actual distance on the earth. This implies that a small scale map represents a large area on the earth and vice versa. 

A map is simply an abstraction of the complex real world.

There are three types of maps. They are listed below:

1. General purpose maps: These maps do not show any feature with special emphasis. They usually show roads, power lines, transportation routes, water features, etc.
2. Special purpose maps: They are made for special purposes such as ocean charts for navigation, cadastral maps to show property ownership details, etc. They are usually of large scale implying that the show a small portion of the earth.
3. Thematic maps: These are maps that have a particular geographic theme. In a GIS, roads, rivers, vegetation, contour elevations, etc are categorised separately and stored in separate map themes or overlays. There are two different types of thematic maps.
1. Choropleth map: These maps contain differential zones. The different zones are used to represent different classes in a theme. Ex: Census tracts, Average income, Percentage female population, mortality rate, etc
2. Isopleth map: These maps contain imaginary lines to connect points of equal value (isolines). These maps can be drawn for variables such as temperature, pressure, rainfall and population density. An example of an isopleth map is a topographic map showing contours.

Maps are classified based on:

-Scale (Large scale, Medium scale and Small scale)

-Content and purpose (Physical maps, Cultural maps)

-Thematic content of GIS coverages (Vegetation maps, Transportation maps, 

Land use/Land cover maps, Remotely Sensed Imagery)

Thematic maps are portrayed as:

-Prism maps

-Choropleth maps

-Point distribution maps

-Surface maps

-Graduated circle maps

-Hydrogeomorphological maps, etc

A 'map legend' links attributes to geographical features

Spatial data is 'graphical'

Aspatial data or non-spatial data is text (Eg: Attributes)

In GIS, the attribute data is loaded into a database and linked to the graphical features

Maps represent SNAPSHOTS OF THE LAND AT SPECIFIC MAP SCALE.

Map portrays three kinds of information about geographic features:

-location and extent of the feature

-Attrubutes of the feature

-Relationship of the feature to the other features (Explicit or Implicit)

GIS distinguishes between spatial and attribute aspects of geographic features.

THE IDENTIFICATION OF RELATIONSHIPS BETWEEN FEATURES, WITHIN A COMMON THEME OR ACROSS DIFFERENT THEMES, IS A PRIMARY FUNCTION OF A GIS

All geographic features on the Earth's surface can be characterised and defined as:

-points,

-lines and

-areas

Point is defined as a single location in space (Eg: (x,y))

Line is described by a string of spatial coordinates (Eg: River, Roadway, Pipeline, etc)

Areas are described by a CLOSED STRING of SPATIAL COORDINATES (Eg: Forests, Soil classification areas, administrative boundaries, climate zones, etc)

Polygon data is HOMOGENEOUS in nature and thus CONSISTENT THROUGHOUT.

Every geographic phenomena can in principle be represented either by a point, line and or area.

-IDENTIFIER accompanies all types of geographic features

-LABELS distinguish geographic features of the same type.

-EACH LABEL is UNIQUE and provides a mechanism for LINKING the feature to its attribute data

-Every feature on the Earth is represented on the map by using a SYMBOL

-The manner in which geographical features (ENTITIES) are represented on the map is DEPENDENT ON SCAPE. This is called SCALE RELATED GENERALIZATION.

-Map scale = (Distance on map) / (Distance on Earth) 

Note: Units for distance in BOTH numerator and denominator SHOULD BE SAME

-REPRESENTATIVE FRACTION = R.F.

-Distance represented and map scale are inversely proportional

-Small scale map shows less detail and covers large area and vice-versa

-Accuracy of a features location is FUZZIER at small scale maps than large scale maps.

-AS SCALE INCREASES, SIZE OF FEATURES DECREASES CAUSING:

disappearance of features

features may change from line to point

features may change shape (less detailed and more generalised)

some features may appear

Application of GIS in Agriculture

Information management plays an important role in improving farming practices and consequently increase agricultural production. In-fact, the practice of agriculture is spatial in nature. The biological and physical aspects of agriculture create spatial variability. Owing to this variability, the  occurrence and distribution of insects along with plant pathogens and diseases that result in decreased crop yield is random. Another word used to describe this variability is, 'patchiness'. Plant disease management practices can be improved by putting epidemiological information and other farm information using a GIS.
Precision farming is based on combining GIS with sophiticated hardware for geographically referenced yield data and variable rate applications of fertilizers and other farm chemicals.
Spatially referenced information provides a gateway to farmers, pest control advisors, extension workers and others to evaluate plant disease problems in a spatial context
GIS relates the data collected by GPS to other sources of geo-referenced information. GIS posseses the ability to integrate layers of spatial information and uncover relationships that were otherwise not obvious.
The process of transforming one layer of spatial information to match a second layer is called registration.
GIS has been applied in agriculture for the spatial analysis of insect pests, weeds and plant diseases.
Since the field of agriculture is primarily spatial in nature, GIS is the appropriate technology to manage the related spatial data.
However, analysis using GIS should be invariably compared with conclusions from a general knowledge of the problem and recheck the methodology and data if there is a major variation with what is generally expected.
GIS risk assessment maps based on a gaussian process regression of field observations were part of a tomato virus management program at the regional scale in Mexico.
Soil structure and chemistry information is linked to a point theme that is displayed as an overlay with field boundaries (polygon features) and roads (line features). The points are colour coded according to the percent total sand in soil. Using GIS, the data can be linked to points using a coordinate system. The 'x' and 'y' coordinates of a point are determined using GPS readings. These points are overlaid on similarly referenced linear and polygon features.
The availability of GIS software, capable of producing attractive maps provides an opportunity to visually communicate plant disease situation to a variety of audiences. GIS output can be used by decision makers to stimulate coordinated action allowing available resources to be focussed on the most significant problems.
GIS is particularly useful in identifying recurring patterns of plant disease and other problems like insect and weed infestations.
The association of environmental factors, landscape features and cropping patterns combined with the recurrence of disease or other problems can be readily communicated to decision makers.

GIS can be used as an agricultural land use planning tool. If soil information is linked from an aerial photograph to a topographic base map and this is added to several other data layers and images like interpretation maps, flood frequency maps and run-off maps, the soil-based GIS would make the decision-making process more accurate, automated and efficient. GIS allows visualization of information in new ways that reveal relationships, patterns and trends not visible easily.
Geographic Information Systems have become an essential and efficient toolkit in all aspects of agriculture from farm management and resource conservation to a wide range of agribusiness applications.
Precision farming is an integrated agricultural management system that incorporates state-of-the-art agronomic knowledge, information from multiple sources, and the global positioning system, geographical information system, yield monitor, variable rate, and remote sensing technologies.

Precision farming allows producers to make management decisions about discrete areas of the field, with the goal of optimizing the crop response based on the production potential and constraints of the specific region. These techniques help in providing good quality land for future generations while preserving the land’s potential for multiple uses, and implement techniques that increase agricultural energy efficiency.
Precision farming can be used to improve fertilizer, seeding, and irrigation rates as well as improved targeting of insecticides and herbicides toward pests. Precision farming techniques are used for improving agricultural efficiency through improved nutrient management. Efficient nutrient use can be improved by adjusting application rates using GIS based precision estimates of crop needs. GIS can be customised to enter soil data and create yield maps. This can be used to  generate fertilizer prescription maps and related reports for site-specific management. GIS is bieng used extensively around the globe as a tool that helps in increased agricultural yield. The spatial analytical tools related to agriculture in GIS are, hydrology parameters, groundwater and data interpolation calculation tools. These tools can be used to adopt efficient sampling strategies and increase profitability. Grid sampling (GIS tool) can be used to reveal soil fertility patterns in farm fields. A latest development in this regard is the Web Soil Survey  (WSS), an interactive online interface providing user defined reports and GIS ready spatial data. Managing data in a GIS ultimately produces a map that helps in visualization of the data.

Precision manure application using GIS and GPS helps farmers to avoid overlapping or missing application areas. GIS also helps in avoiding application of manure in environmentally sensitive areas, turning the applicator off when stepping out of the boundary area and varying the application rate based on projected crop nutrient needs at different locations across fields. The value of manure as a crop input is diminished by improper application. While lack of application of manure causes nutrient deficiency resulting in reduced crop yields, excessive application of manure also results in low crop yield due to excess vegetative growth resulting in increased crop diseases. Geospatial technologies (GIS, Remote Sensing and GPS) can help in implementation of efficient manure management practices like,

1. determining optimum amount of manure to apply at specific locations in fields for specific crops and yield goals, 
2. applying prescribed rates and 
3. when and where manure was applied.

GIS has been used as a decision support tool for a soil-based automated variable rate irrigation sprinkler system. GIS has also been used for soil salinity mapping. GIS can be used to create Soil Organic Carbon (SOC) maps. Thus, site-specific management and assessment would be very difficult if not impossible without GIS mapping and tools. GIS tools can be used to improve distribution of manure nutrients at the national, local and field scales as a means to minimise environmental contamination. GIS plays an important role in managing nutrient imbalances. GIS can be used in conjunction with modelling to understand how crop selection and soils interact to effect environmental outcomes across an agricultural landscape. For example:

1. a public drinking water supply faced with increasing nitrate in groundwater source and
2. planning of a new biomas conversion facility to produce renewable fuels from grain or cellulosic feedstock. 

In the above cases GIS can be used to help the audience gain new understanding of the problems and visualise how to circumvent the problem by growing different crops. In the first case, a soil map can be used to extract soil characteristics and individual polygons of GIS coverages are used to produce final map products depicting areas of high, moderate and low nitrate leaching risks.