SRTM Terrain Database

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The EGPWS for our simulator was already developed some time ago. You can read all about it here.

It was originally developed for use with FSX and obtains its terrain elevation data from FSX through an own developed probe scan technique.

With the addition of Xplane 11 to the A320Sim Solution this technique is however not available anymore, so we had to go for an alternative approach to provide the EGPWS with terrain elevation data. The perfect candidate was SRTM...

SRTM  - Shuttle Radar Topography Mission

The Shuttle Radar Topography Mission (SRTM) was flown aboard the space shuttle Endeavour in February 2000. It collected radar data over 80% of the Earth's land surface between 60° north and 56° south latitude with data points posted every 1 arc-second (approximately 30 meters). The data has been made public in late 2015 and since then has been further processed and enhanced to fill voids, remove spikes, etc. It is available in various spacial resolutions, the most common of which are 3 arc-seconds (~90 meters) and 1 arc-seconds (~30 meters) for global coverage.

At these resolutions the data size is however huge and the EGPWS can not process that amount of data in realtime during the flight. So we developed an SRTM terrain database parser that offline can do pre-processing of the high resolution data and that stores it in a highly condenced file format at a resolution of 1000 meters. During flight the EGPWS then dynamically reads the pre-processed terrain elevation data and displays it in realtime on the Navigation Display (ND). Below you can see the output of the SRTM reader/pre-processor that has been developed, showing the whole World and a zoom in on Europe.

SRTM Reader - World
SRTM Reader - Europe

EGPWS - comparison of results


LSZB (Bern, Switzerland)

Let's compare the results of the FSX Probe Scan method and the new SRTM Terrain Data based approach which can be used for Xplane 11 (and also for FSX obviously).

We placed our A320 at LSZB (Bern - Switzerland) for Takeoff at RWY 14. On the left you see the ND with FSX running and using the FSX Probe Scan data and on the right you see the ND with Xplane 11 running and using the SRTM Terrain Elevation data.

FSX using Probe Scan data

EGPWS - LSZB 14 ND - FSX

Xplane 11 using SRTM data

EGPWS SRTM - LSZB 14 ND with border

This is what Google Maps shows. It is a pretty good result 🙂

EGPWS - LSZB 14 Google Map - Zoom

 

LSGS (Sion, Switzerland)

As a second comparison we placed the A320 at LSGS (Sion, Switzerland) for Takeoff at RWY 25.

On the right hand side you see the ND terrain data from the EGPWS using the SRTM Terrain Elevation database.

This time we zoomed the ND in on its shortest range.

Below you see the Google 3D birds eye view looking into the direction of the runway heading.

Again a very nice result.

Xplane 11 using SRTM data

EGPWS SRTM - LSGS 25 ND with border - large

LSGS (Sion, Switzerland) RWY25 - Google Map 3D

EGPWS - LSGS 25 Google Map 3D

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