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I am new to GIS.

I am using s2p stereo pipeline to build Digital Surface Model (DSM.tif) from it. The height of certain point in this DSM.tif is 84 meters, and you can see from the color bar on bottom left of this DSM.tif, the average height of this area is about 84 to 130 meters. Below is DSM.tif. enter image description here

However, I use QGIS3 plugin SRTM downloader to download a map within this area. And the average height of this area is only 45 meters. Below is SRTM model. SRTM model

My question is why is this happening?

Is there a height translation between this two model?

I dive into the input RPC.XML file, but not really understand it. Here is the RPC.XML. I found <HEIGHT_SCALE>75</HEIGHT_SCALE> <HEIGHT_OFF>125</HEIGHT_OFF>.

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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