Terrain Elevation Data Usage

SoarPilot has the ability to load and read terrain elevation data in order to provide a display of the current terrain elevation as well as an “above ground” (AGL) altitude approximation value. It also allows Soarpilot to check if any terrain will interfere with your glide path to your target waypoint, or around your task if you have one active. See the Final Glide Over Terrain option for details.

This terrain data will come from a .pdb file that you will have to hotsync into your PDA (explained below). When the program is running, it uses this terrain elevation data to determine the elevation of the ground at your current location. You can see the current terrain elevation on the Flight Info screen. (Note: The moving map screen does not show a coloured terrain map. This is to keep the display as clear as possisble and maximise the contrast for readability in strong sunlight) The upper part has a new field which is always updated with this info. If there is no terrain data loaded or you have traveled off of the loaded data, “N/A” will be displayed in the field. In addition, the current terrain elevation is saved with the current flight information.

pia03396_small.jpg
Old (GTOPO30) New (SRTM30)

The terrain information originates from GTOPO30-formatted data which is readily available via the internet. GTOPO30 is a global digital elevation model (DEM) with a horizontal grid spacing of about 30 arc seconds which equates to about 1 km. I’ve played around with looking at several places both in the UK and in the US comparing the GTOPO elevation with the reported field elevation for the airfield. With a 1km resolutioin, it’s not perfect of course but it does seem to average around 10-20 feet of the actual value for the airfield I checked. That’s not too bad I think. Actually putting in support for reading this information was not too difficult. Also because of the way the data is laid out (every grid square is .0833333333 of a degree on each side) it is a simple/fast calculation to take the current lat/long and go directly to the correct row and column in the terrain file for the elevation information. While a PalmIIIx may not have enough RAM to hold the data it would certainly in no way make the unit any slower. It was very nice that it worked out that way. What took longer though was creating a PC-based conversion program to read in the .DEM information which is HUGE (40deg x 50 deg = ~57Meg), allow people to specify a smaller area (and thus smaller size) and then package that into a .pdb file which can be easily loaded and used by SoaringPilot.

To support this, there is a new version of SPTerm (1.3.3) available through the webpage as well. When you run the new version of SPTerm you will find there is a “;Terrain” menu item now with one menu item which says “Generate”. Selecting this option will open the terrain generation window. The first field on the screen allows your to enter the filename of the terrain .pdb file to be outputted. By default it suggests a filename of “SoaringPilot_terrain_db.pdb” and I would suggest not changing that name. However it really doesn’t matter what you call it because the internal filename is the only one that matters when loading it on the Palm and that name is hardcoded to the proper value. This allows you to have multiple terrain .pdb files with different names to differentiate the area they correspond to. Then you just have to load the one you want to use before you head out to the airfield.

Next, you must enter the upper and lower latitudes’s and the left and right longitude’s, both in decimal degrees with S and W being negative. Then when you select the Generate button, it will produce the .pdb file in the “terrain” directory. However, this will of course give you an error when you first try this because you won’t have the required .dem data. However, it will create the required “terrain” directory under the SPTerm install directory.

To download the GTOPO30 data and make things work properly, you will need to go to:

http://eros.usgs.gov/#/Find_Data/Products_and_Data_Available/gtopo30_info
Mirror: http://www1.gsi.go.jp/geowww/globalmap-gsi/gtopo30/gtopo30.html

click on the sector(s) you want to download and do so. Each sector you download will be a .zip file of about 8 or 9 Meg. Inside these zip file are several files however you only need to extract the .dem file into the “terrain” directory under the SPTerm directory. This is the largest file in the .zip file and is about 57Meg. If the area you want to define for terrain information covers more than one sectors/.dem file, you will need to download all required files and place the .dem files into the terrain directory. (NOTE: If you use WinZip please see the note on the download screen for each sector. If you are using WinZip to uncompress the .dem file, please turn off (uncheck) the TAR File Smart conversion found under Options / Configuration (or under Misc. if using Winzip 8.0) The terrain generation allows using a maximum of 4 .dem files. Once the .dem file(s) is in the terrain directory, the generator will work properly and will place the newly created .pdb file in the terrain directory as well. You simply have to hotsync this .pdb file onto your Palm PDA.

Finally, as a result of NASA’s SRTM (Shuttle Radar Topography Mission) last year, a new topographical dataset is also available in the same GTOPO30 format. It is called SRTM30. SRTM data was used to update the older USGS GTOPO30 global DEM, by averaging the data to 30 arc-sec resolution and replacing GTOPO30 heixels between the latitudes of 60º North and 56º South. The resulting data can be directly downloaded and is now the best available global elevation dataset. The above examples screenshots show the improvements. The left image is the old data. The right is after being updated with SRTM data. As you can see, the right is much more accurate. This is because SRTM30 replaces the wild guesswork of previous data with actual measured values.

The new data uses the same names as the previous GTOPO30 data so once you find the filename you want to download from the GTOPO30 site, go to the following URL and download the same file:

http://dds.cr.usgs.gov/srtm/version2_1/SRTM30/

Related Pages

 
soarpilot/terrain_elevation_data_usage.txt · Last modified: 2010/02/25 14:43 by amegens
 
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