Digital Elevation Model (DEMs) from Topographic Maps
|The process of Digital Elevation Model (DEM) creation begins with the scanned and geo-referenced topographic map (or nautical chart in the case of bathymetry). Contour Lines, spot elevations, coastlines and bodies of water from the raster image are extracted, converted to digital vectors and given elevation values.
Once a digital image has been fully vectorized and attributed, a raster representation of the map is created based on those vector features. After editing and testing the resulting file is then output based on the clients file format and projection specifications.
Contact LAND INFO for a project recommendation.
Azerbaijan 20m Digital Elevation Model (DEM) custom built from over 300 1:50,000 topographic map sheets.
USGS ASCII (.dem)
• ESRI GRID
• ESRI BIL with HDR
• Digital Terrain Elevation Data (.dted)
• Generic ASCII
• Generic BIL
• ERDAS Imagine (.img)
• ER Mapper (.ers)
|DEM file sizes
will largely depend on the scale and contour interval of the source
map, file format, and the spatial resolution of the DEM. DEMs generally
compress quite nicely due to the file format and structure of the
||Land Info provides
several different delivery options:
• USB flash drive
• External hard drive (USB 2.0/USB 3.0/FireWire 400/FireWire 800/eSata)
Each DEM goes through a variety of steps to achieve
the desired quality that Land Info has delivered time and time again.
- The vector data is directly compared to the
Topographic Map to ensure the lines accurately represent the source
map and to verify the vectors have the proper elevation values.
- Benchmarks are used to check the accuracy
of the attributes and to check the elevation values of contour lines.
When coastlines exist, they are also used in this regard.
- The DEM goes through an edge matching process
that verifies a visibly smooth transition from one DEM (one map sheet)
to another. This portion is done manually to assure the information
from the source Topographic Maps are transferred to the DEM and there
is no loss of accuracy.
- Each DEM must pass a Root Mean Square Error
(RMSE) check. An RMSE check assures that the DEM is accurate in comparison
to the source Topographic Map by comparing a DEMs elevation values
to estimated horizontal and vertical values and control points that
have accurate elevation data associated to them. The information derived
from the RMSE check is recorded in the “C” record, which
is an element of the standard DEM output file format.
The control points are an important component
of the DEM Quality Assurance check. Control points range from spot heights
to points along index contours. A minimum of 30 control points selected
throughout the entire file. Several different elevation values are used
to spread the checked points evenly throughout the entire map. Several
of the 30 points will be chosen near the edges and corners of the DEM
to account for edge matching. Two RMSE tests and an average deviation
test are performed to ensure the quality of the DEM. All three of the
tests outcomes should be less than half the value of the contour interval
of that specific map.
The following is an example of a typical
Contour Interval = 10 meters
Number of Control Points Selected = 38
RMSE0 = 4.51
RMSE1 = 4.57
Average Deviation = 0.07
The Spatial Resolution (otherwise known as Grid
Posting) of a Digital Elevation Model highly depends upon two different
factors - contour interval and Topographic Map scale. The contour interval
of a Topographic Map will vary depending on the lay of the land and
the amount of detail that can be represented at any given Topographic
Map scale. This chart shows some typical spatial resolutions from different
Topographic Maps: (Note: Different cell sizes can be used for different
scales of Topographic Maps, however this could result in a loss of accuracy.)
||1/3 Arc Second
||1 Arc Second
||2 Arc Second
||3 Arc Second
||3 Arc Second
• Examples of Digital Elevation Models (DEMs)
• Digital Elevation Models (DEMs) Information
• Digital Elevation Models (DEMs) from Satellite Imagery