9.9Digital Elevation Models (DEM) FAQ and Tips

This section goes over the implementation of DEMs into GrafNav and how to obtain some DEM data for the United States over the Internet.

DEMs are optional when processing GPS data.

A Digital Elevation Model (DEM) is a representation of the height of the ground. There are two ways they can be stored:

1.Regular grid with constant density that is, spacing. The DEM is stored as an array to minimize memory usage. Access is very fast, but flat areas require an unnecessarily high number of points.

2.Irregular point distribution where density generally increases with increased relief. This type is represented by a Triangulated Irregular Network (TIN).

While it can theoretically handle both types, GrafNav uses the latter type. The first one will be handled much less efficiently.

DEMs, after loading, are expressed in ellipsoidal height. Therefore, if the DEM is mean-sea-level, which is usually the case, the geoid undulation must be subtracted from the elevation values. This is performed automatically during the import process but it requires that a geoid (WPG) be available.

Tip: GrafNav operates best when contour plotting is disabled. This process can be very slow and can cause problems with the interface.

9.9.1Why would I use a DEM?

There are two main reasons for using GrafNav’s DEM capabilities:

1.The vehicle is traveling over a surface (for example, road, runway, and so on) and the ground height is to be used for quality control purposes. GrafNav has the following plotting tools that help:

•The Height Profile plot shows the height of the ground as well as remote station height. If the GPS antenna is a constant vertical offset above the ground, it should be entered as the remote antenna height.

•The Height Above Ground plot shows the difference between kinematic GPS and ground heights. The ground surface height at each GPS position can be exported as well. DEM precision and accuracy is very important here.

2.In aerial photogrammetry, mapping is often performed on the map projection. This leads to an incompatible scale used for horizontal and vertical axes. The vertical scale factor (VSF) correction compensates for this, but usually only accepts a constant ground height. A more accurate compensation can be employed by using the height of the DEM under each image frame. In high relief areas, the DEM can improve this correction significantly. The amount of improvement depends on the size and variation of the scale factor in the project area and variation of the ground height. DEM accuracy is not as important, given that a 25 m DEM error will result in a very small final error in VSF height for UTM and an insignificant error for most State Planes.

9.9.2What are the DEM sources?

There are many sources, but for VSF correction purposes, 1:250 000 (1-degree) DEMs are sufficient. In the USA, these can be obtained free of charge from the Internet, via the following steps:

1. Go to http://edc2.usgs.gov/geodata/index.php and click on the 1:250,000 Scale Digital Elevation

Models along the top. Select Alphabetical List.

2.Navigate the directories to find the area of interest. Download the map sheets from USGS site.

3.For each file, use the utility DEM2XYZN.exe, available from http://data.geocomm.com/dem/ dem2xyzn/, to convert these files to a listing of geographic points. The output will be Longitude Latitude Height where the latitudes and longitudes are in arc-seconds and the height is in metres.

4.If needed, concatenate files of together to create one file that will cover the entire project area. From the command prompt, use the Copy command and the following convention:

•copy file1.xyz + file2.xyz + file3.xyz combined_file.xyz

See Section 2.6.6, on Page 99 for help importing the DEM file into the software.

9.9.3What DEM formats are supported by GrafNav?

GrafNav only supports DEMs stored in ASCII, where each elevation point is a separate record in the file. The full position must be made available for each point. This means that many grid files must be expanded before importing. The following formats are supported:

The [ID] term is an optional identifier that may be present. The last format listed is the one used in those files created by the DEM2XYZN utility.

9.9.4How do I handle large DEMs?

DEMs can become very large. GrafNav’s ability to handle them is very functional but is not designed for extremely dense data sets. DEMs consisting of over 500 000 points will become very slow within the software. They will also require a significant amount of memory. Such a DEM will utilize an additional 157 MB in GrafNav, not including contours. This means that a fast computer with at least 512 MB of memory is required. For VSF applications, DEM accuracy is not paramount, so DEM thinning is highly suggested.

Some helpful hints to speed up GrafNav with large DEMs:

•Enabled the Reduce DEM density using error tolerance option during the importing of the DEM procedure. This will cause the TIN formation to take longer, but GrafNav will operate much quicker afterwards. This is especially important for regular grids.

•Before importing, create a GPS trajectory composed of just the project area. Use the start/end processing times to achieve this. Ensure that the Reject points outside DEM area option is enabled.

•Disable contouring under Settings | DEM Plotting, or else select a very coarse contour interval. See Section 2.6.6, on Page 99 for more information.

•Disable ground height plotting on the Height Profile plot under Settings | DEM Plotting. See Section 2.6.6, on Page 99 for more information.

•In GrafNav Batch, only load the DEM into one baseline because an instance of the DEM will be created for each baseline.