Hypsography, topography, elevation, TINS, and contours. There are many different terms for data relating to elevation. This article reviews some of terminology and types of elevation based datasets available to GIS. Digital elevation data are sets of elevation measurements for locations distributed over the land surface.
Elevation data have many practical uses ranging from environmental to urban. Slope and aspect can be directly derived from elevation. Stream delineation and subsequently watershed boundaries can also be derived. Elevation datasets are also used for scenario analysis ranging from calculations of cut and fill requirements by engineers for projects relating to road construction to viewshed analysis. Viewshed analysis (or line-of-sight) uses topographical data to determine the visibility of areas from a given point. For example, a project in which a scenic route is to be constructed may utilize elevation to determine the visibility of the landscape from various points to determine the best pathway to construct.
Elevation data can be generated from existing contour maps, photogrammetric analysis of stereo aerial photography, satellite imagery (as described in the STRM article), or laser flights.
Elevation values are most commonly shown relative to sea level. Thus positive values indicate areas found above sea level and negative values are places on the earth found below sea level.
Elevation Data Types
There are three common methods to depict elevation data to create a statistical surface:
- Regular Grid
- Triangulated Irregular Network (TIN)
There are many terms for the different types of regular grids representing elevation data sets:
DTD – Digital Terrain Data
DEM – Digital Elevation Model
DTM – Digital Terrain Model
DTED – Digital Terrain Elevation Data
All of these terms are synonymous with DEM being the most commonly utilized term. DEMs stored elevation value in regularly spaced intervals. Each value represents the elevation value of that cell.
Digital elevation model (DEM)
DEMs are used for a variety of spatial analysis. The most common datasets derived from DEMs are slope, aspect, hydrology and watersheds.
The main limitation of DEMs is that they are regularly spaced. This results in excess data storage in areas of low relief and loss of detail in areas of high relief.
Triangulated Irregular Network (TIN)
One approach to the problem of data file size is through Triangulated Irregular Network (TIN). A TIN is a vector-based topological data model used to represent terrain. As indicated by the name, TINs contain a network of irregularly spaced triangles. Areas of high-relief will contain a higher density of small triangles while areas of low-relief will be represented by larger triangles. TINs can store a higher amount of topographical detail in a smaller sized dataset.
Contours refer to lines representing equal points of elevation on the surface of the earth. A synonym, but less frequently used word is hypsography. This word is derived from the Greek “Hypso” meaning height.
Contour intervals reference the vertical distance between lines. For example, 10-meter contours indicate that a 10-meter rise or fall in elevation exists between two adjacent lines. The distance between lines indicates slope: areas of dense linework will indicate steep terrain. The variability in topography is referred to as relief. Relief refers to the vertical elevation differences in the landscape. Low relief refers to the landscapes that have very little changes in elevation such as plains. High relief describes a landscape with extreme changes in elevation as one might find in the Rockies or the Himalayas.
Finding Elevation Data
There are many resources available on the Internet for finding elevation datasets to use. The best known source of free and low-cost DEMs is the USGS. The widely publicized Endeavour mission this past year was most well-known for its SRTM mission which sought to gather topographical data for over 70% of the Earth’s surface.