Introduction to Cartography - Lecture 13


Isarithmic Maps
  • are three-dimensional volume maps
  • the three main models are: 1) surface, 2) relief, and 3) planimetric/contour
  • isarithmic maps can be based on point data (isometric) or contours occurring over geographical regions (isoplethic)

Advantages Of Isarithmic Maps
  • have a good 'feel' to them >>> readers like to look at them
  • offer a "third-dimension" for an additional vantage point
  • illustrate areas of peaks and valleys across a surface

Disadvantages Of Isarithmic Maps
  • the "third-dimension" can make interpretation difficult for the novice reader
  • extrapolation, interpolation, and kriging techniques can introduce errors that can not be identified by the reader
  • can be difficult to produce if the proper output device is not available
  • legends and visual anchors can be difficult to generate for surface and shaded relief models

Digital Terrain Models (DTMs)
  • are isarithmic models generated from point and contour data
  • represent a model of an Earth surface, elevation and/or bathymetry
  • digital elevation models have been produced by the U.S. Geological Survey at scales of 1:24,000 and 1:250,000 for most of the United States
  • perspective viewing of DTMs can be changed by altering the interval or datum used to generate the model
  • viewing angle and sun angle can also be utilized for a different viewing perspective

Geographic References For DTMs

spot elevations - point locations measured by the Geodetic Survey
bench marks - precisely measured point elevations used for reference
soundings - point locations of bathymetry


Planimetric/Contour Maps And Models

Types of Contours

Isolines - lines of constant value
Isobath - depth contour
Isohypse - elevation contour
Isotherm - temperature contour
Isohyet - precipitation contour
Isogone - contour of magnetic deviation


Planimetric/Contour Maps And Models

contour interval - the difference between adjacent contour lines, e.g., the contour interval for a USGS 7.5' quad would be 5 feet if the elevation contours ran shoreline, 5', 10', 15', 20', etc.

hachured contours - are contours that show areas of depression

form lines - are used to indicate contours where data may not be available or reliable

supplemental lines - used to indicate areas where a slight change of relief may cause dramatic environmental hazards, e.g., flood plains

index contours - contours that are labeled for elevation or depth; used for guides in analysis

shadowed contours - utilized to create an impression of depth

superimposed contours - when contours are overlaid on photographs, images, or DTMs


Relief Maps And Models

  • main types are: 1) smooth shaded, 2) plastic shaded, and 3) fish-net perspective
  • these are computer generated
  • shaded models incorporate a 'light source' to effectively create a dimensional perspective
  • stepped continuous or smooth continuous are the methods for representing these models
  • when known datums are utilized, relief models are termed 'absolute'