Doubt on the external DEM usage

[riyasmjgeo]

Hello All,

I am using REMA DEM in ISCE2. I have converted the DEM to ISCE format, and I am able to perform topsApp computation with it. However, when I crosscheck the input DEM and the DEM ISCE derived (*.wgs84 format), I found it strange that most parameters are comparable, but the mean of the DEM is significantly different. Considering the same CRS in both, why should it have this much difference in the mean?
Am I missing something?
I gave this DEM as the parameter demfilename. Instead, am I supposed to assign my input directly as geocode demfilename in topsApp input XML?

### Metadata of input DEM ### 
Driver: ISCE/ISCE raster
Files: rema_isce_crop
       rema_isce_crop.aux.xml
       rema_isce_crop.xml
Size is 14732, 4795
Coordinate System is:
GEOGCRS["WGS 84",
    DATUM["World Geodetic System 1984",
        ELLIPSOID["WGS 84",6378137,298.257223563,
            LENGTHUNIT["metre",1]]],
    PRIMEM["Greenwich",0,
        ANGLEUNIT["degree",0.0174532925199433]],
    CS[ellipsoidal,2],
        AXIS["geodetic latitude (Lat)",north,
            ORDER[1],
            ANGLEUNIT["degree",0.0174532925199433]],
        AXIS["geodetic longitude (Lon)",east,
            ORDER[2],
            ANGLEUNIT["degree",0.0174532925199433]],
    ID["EPSG",4326]]
Data axis to CRS axis mapping: 2,1
Origin = (50.192850475016627,-64.301526245846318)
Pixel Size = (0.002500818414793,-0.002500835787499)
Metadata:
  AREA_OR_POINT=Area
  DataType=Generic
Corner Coordinates:
Upper Left  (  50.1928505, -64.3015262) ( 50d11'34.26"E, 64d18' 5.49"S)
Lower Left  (  50.1928505, -76.2930338) ( 50d11'34.26"E, 76d17'34.92"S)
Upper Right (  87.0349074, -64.3015262) ( 87d 2' 5.67"E, 64d18' 5.49"S)
Lower Right (  87.0349074, -76.2930338) ( 87d 2' 5.67"E, 76d17'34.92"S)
Center      (  68.6138789, -70.2972800) ( 68d36'49.96"E, 70d17'50.21"S)
Band 1 Block=14732x1 Type=Float32, ColorInterp=Undefined
  Min=-37.933 Max=2833.312 
  Minimum=-37.933, Maximum=2833.312, Mean=1239.592, StdDev=827.958
  NoData Value=nan
  Metadata:
    RepresentationType=ATHEMATIC
    STATISTICS_MAXIMUM=2833.3115234375
    STATISTICS_MEAN=1239.5924194365
    STATISTICS_MINIMUM=-37.932842254639
    STATISTICS_STDDEV=827.95848714651
    STATISTICS_VALID_PERCENT=13.86

---

### Metadata of the DEM derived ###
Driver: ISCE/ISCE raster
Files: rema_isce_crop.wgs84
       rema_isce_crop.wgs84.xml
Size is 14732, 4795
Coordinate System is:
GEOGCRS["WGS 84",
    DATUM["World Geodetic System 1984",
        ELLIPSOID["WGS 84",6378137,298.257223563,
            LENGTHUNIT["metre",1]]],
    PRIMEM["Greenwich",0,
        ANGLEUNIT["degree",0.0174532925199433]],
    CS[ellipsoidal,2],
        AXIS["geodetic latitude (Lat)",north,
            ORDER[1],
            ANGLEUNIT["degree",0.0174532925199433]],
        AXIS["geodetic longitude (Lon)",east,
            ORDER[2],
            ANGLEUNIT["degree",0.0174532925199433]],
    ID["EPSG",4326]]
Data axis to CRS axis mapping: 2,1
Origin = (50.192900000000002,-64.301500000000004)
Pixel Size = (0.002500820000000,-0.002500840000000)
Corner Coordinates:
Upper Left  (  50.1929000, -64.3015000) ( 50d11'34.44"E, 64d18' 5.40"S)
Lower Left  (  50.1929000, -76.2930278) ( 50d11'34.44"E, 76d17'34.90"S)
Upper Right (  87.0349802, -64.3015000) ( 87d 2' 5.93"E, 64d18' 5.40"S)
Lower Right (  87.0349802, -76.2930278) ( 87d 2' 5.93"E, 76d17'34.90"S)
Center      (  68.6139401, -70.2972639) ( 68d36'50.18"E, 70d17'50.15"S)
Band 1 Block=14732x1 Type=Float32, ColorInterp=Undefined
  Minimum=-13.048, Maximum=2859.581, Mean=190.829, StdDev=529.665
  Metadata:
    STATISTICS_MAXIMUM=2859.5812988281
    STATISTICS_MEAN=190.82894473731
    STATISTICS_MINIMUM=-13.047580718994
    STATISTICS_STDDEV=529.66509328436
    STATISTICS_VALID_PERCENT=100

[EJFielding]

Hello Riyas,

When the ISCE2 software converted the REMA DEM elevations to WGS84 ellipsoid heights, it will change the mean height because the reference is different. The original data is relative to the geoid.

[riyasmjgeo]

Thank you for the response @EJFielding . If I am not wrong, according to the metadata, the input itself is in WGS84 ellipsoid rather than geoid. Please let me know if I am wrong.
In case I am right, am I supposed to assign the DEM as geocode demfilename rather than providing as demfilename input?

[EJFielding]

If your input DEM is already in ellipsoid heights then you need to tell ISCE that in your xml file. I forget the exact syntax, but you can look at a dem created by ISCE for another area in the "dem.wgs84.xml" file.

[riyasmjgeo]

Thank you.