Hypsography, topography, elevation, TINS, LiDAR, shaded relief, 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.
What is elevation data?
Elevation data provides information about the height, or altitude, of points on the surface of the Earth relative to a specific vertical datum, usually sea level. For the most part, positive values from elevation data shows areas above sea level, and negative values depict areas below sea level or vertical datum.
Elevation data is cross-functional and may come in different forms to serve disciplines like geography, geology, urban planning and works, environmental monitoring and management, among many others.
Uses of elevation data
Elevation data have many practical uses ranging from environmental to urban. Slope and aspect can be directly derived from elevation. 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 to collect LiDAR data.
During terrain modeling, slope, aspect, and curvature is derived from elevation data to provide insight on terrain characteristics for geological studies, landform classification, and suitability modeling.
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.
In viewshed analysis and studies, elevation data determines the areas visible from specific viewpoints. This information is crucial for siting structures like communication towers, wind turbine, and observing natural features. 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.
Streams, hydrological networks, and watersheds can be derived from elevation data to model water flow and drainage patterns, simulate and predict the behavior of floods under different conditions, and provide emergency response planning and floodplain management.
Elevation data enhances navigation systems by providing altitude information, especially in aviation and outdoor activities such as hiking and mountaineering. The dynamic and cross-functionality of elevation data enables well-informed decision making, precise modeling, and a savvy understanding of the Earth’s surface and eco-structure.
Elevation values
Elevation values are most commonly shown relative to sea level. Thus positive values are, for the most part, used to indicate areas found above sea level and negative values are places on Earth found below sea level.
Creating elevation data
Geo-information analysts and environmental management officials subscribe to methods of elevation data by having accuracy, coverage, level of detail, resolution, and applicability in mind. These factors ensure that the elevation data they subscribe to aligns with their project goals and requirements.
Elevation data types
There are three common GIS methods to depict elevation data to create a statistical surface:
- Regular Grid
- Triangulated Irregular Network (TIN)
- Contours
Regular grids – elevation data
Regular grids may come in different forms to provide a structured and systematic way to represent elevation data, enabling efficient analysis and visualization of terrain characteristics. Among the several types of Regular grids like Digital Terrain Data (DTD), Digital Elevation Model (DEM), Digital Terrain Model (DTM), and Digital Terrain Elevation Data (DTED), the most widely used is DEM.
DEM provides a digital representation of the Earth’s surface, created using data sources like satellite imagery, Light Detection and Ranging (LIDAR), and stereo aerial triangulation through a grid of elevation values, with each cell representing a specific location on the ground.
A notable demerit of DEM data is that they require large storage space and computational resources and may vary in quality based on the resolution and accuracy of the data collection methods. Additionally, interpreting DEM data requires specific technical expertise and may require processing and adjustment to correct errors and artifacts in the data.
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
DEMs stored elevation value in regularly spaced intervals. Each value represents the elevation value of that cell. DEMs are used for a variety of spatial analysis. The most common datasets derived from DEMs are slope, aspect, hydrology and watersheds.
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.
TIN answers questions on large storage and computational resources as it is a vector-based topological model that uses irregularly distributed triangles. As a result, TIN offers advantages in accurately representing complex terrains. The key advantage of a TIN over other methods of surface representation, such as a raster grid, is its ability to conform precisely to the surface’s complexity in areas where more detail is needed, and to generalize in areas where less detail is sufficient.
However, potential problems are associated with interpolating values for unmeasured points within the triangles.
Topographical contours
Situations of aesthetically pleasing maps that convey topographic information while maintaining artistic quality may require contours. Contours consist of lines that connect points of equal elevation, allowing you to visualize the shape and steepness of terrain features.
Contour lines are widely used on maps to provide a clear understanding of topography in a simple manner and provide valuable information about the terrain’s relief, showing where the land rises, falls, or remains relatively level.
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.
What is relief in elevation data?
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 USGS offers access to digital elevation models at varying levels of resolution ranging from project-based elevation data that varies from one to five meters. The USGS also offers seamless DEMs cover the larger geographic area of the United States with resolutions of: 1/3 arc-second, 1 arc-second, and 2 arc-second. The coverage of the 48 conterminous states, Alaska, Hawaii, and U.S. territories, as well as parts of Canada and Mexico depends on the dataset.
Elevation data of specific geographic locations can be accessed freely by combining Google Earth Pro and tools from the GPS Visualizer’s website. To find the elevation of a specific geographic location, GPS Visualizer’s website uses Google data to make it possible by accepting coordinates of that location.
Digital Elevation Models (DEMs) like SRTM, the National Elevation Dataset (NED), and other elevation data products are available from the United States Geological Survey (USGS) once a free sign-up process is completed. Elevation data may be searched for, retrieved, and visualized using the Earth Explorer platform offered by the USGS.
This article was originally written on August 27, 2033 by Caitlin Dempsey and has since been updated.
