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
