Wednesday, July 15, 2015

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
      1. 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