How to interpret the height of output DSM.tif?
lionlai1989 opened this issue · comments
Hi guys:
I am a newbie in GIS, so please bear with my newbie question. I've read this issue about the height reference, but not completely understand it.
Let's say now I have a DSM.tif from s2p pipeline. As you can see in the DSM.tif below, the height of certain point in this DSM is 84 meters, and you can see from the color bar on bottom left, the average height of this area is about 84 to 130 meters (Because it's mostly green and yellow).
However, I use QGIS3 plugin SRTM downloader to download a map within this area. As you can see below, the average height of this area is only 45 meters.
My question is that why is this happening? Is there a height translation between this two model? This issue says that the s2p use height reference in RPC.XML to determine the height of DSM.tif, but I am not fully understanding it.
I dive into my input RPC.XML file below. According to the explanation in the reference issue, I found <HEIGHT_SCALE>75</HEIGHT_SCALE> <HEIGHT_OFF>125</HEIGHT_OFF> in my RPC.XML. What does it mean? Does it mean that the height difference of the lowest point and the highest point is 75 meters in this image. And the lowest point in this image is 125 meters of Ellipsoidal height. If this is true, then how do I combine the height values of these two model to get a more accountable result?
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Any help would be appreciated.
Best Regards,
I think I found the answer of part of my question. According to this article, <HEIGHT_SCALE>75</HEIGHT_SCALE> <HEIGHT_OFF>125</HEIGHT_OFF> means that the ellipsoid height is varied from -50 to 200 meters. So s2p uses these two value to calculate the DSM. Therefore, the height in DSM.tif is ellipsoid height. However, I still don't understand why the height in DSM.tif varies a lot than the height of downloaded SRTM elevation model. Can someone give me some insights about this? Or what parameters I can try to make the height difference between DSM.tif and DEM smaller? E.g. using orthorectified image or something like that?
Thank you.
Hi,
I would check the datum (reference surface) for your SRTM (ellipsoid or geoid).
This is because usually RPC's produce heights with respect to the ellipsoid,
but srtm is usually wrt the geoid.
best
Thank you @gfacciol . Do you need further information from me to do that?
Best Regards,
Hi @gfacciol
I found the following information.
DSM.tif: EPSG:32632 - WGS 84 / UTM zone 32N - Projected
SRTM DEM: EPSG:4326 - WGS 84 - Geographic
Best Regards,
This issue is closed because I found the answer. The SRTM is in geoid.