4.03.2011

How to prepare elevation range of SRTM in ArcMap for 3d visualization in ArcScene

If you have a DEM, you might want to view it in 3D. For me, I do geovisualization in ArcScene, VTP or for simple presentations I use Google Earth.

DEM or digital elevation models contain the heights of our terrain. It has a commonly interchanged counterpart term known as DTM (digital terrain model). DEM differs from DTM in the sense that DEM is not the elevation from the ground per se. It is the height with the buildings and other cultural features. DTM on the other hand gives the height of the bare ground.

For DEM, we can obtain freely available datasets such as SRTM and ASTER. SRTM resolution is 90m X 90m for the Philippines while ASTER is 30m x 30m. Though ASTER is of higher ground sampling distance than SRTM, researches show that SRTM still proves to be more accurate than ASTER. 

So for the purpose of visualizing our terrain in 3d. What we would first want to do is to prepare the DEM (i.e. SRTM DEM ) for processing. If we fail to do this, we will be confronted with a very large range of elevation values that ArcScene or our visualization software can't reconcile and we end up not seeing anything on our screen.

So here are the steps for doing it in ArcMap.
Example: if we have the DEM named ISABELA, run these codes in the Spatial Analyst> Raster Calculator.

temp = con ( [ISABELA] >= 32768 , [ISABELA] - 65536 , [ISABELA])

temp1 = setnull ( [temp] < -9998 , [temp] )

to remove the NoData, run the following commands
temp2 = Con( isnull( [temp] ), focalmean( [temp1] , rectangle, 5, 5 ), [temp1] )
 temp3 = Con( isnull( [temp2] ), 1, 0 )
The resulting grid is in floating point format so convert it back to integer using,
Final = Int( [temp3] ) 
Make the Final GRID DEM permanent by right clicking Final layer> Make permanent then choose location and file name.

You may also opt to export it in other formats such as geoTIFF for use with other visualization softwares. You can easily do this by right clicking the DEM GRID> Export Data then choose type of raster, path and 13-character filename.

2 comments:

  1. Hi! :)
    Just a question..
    How will I know exactly know what SRTM data (.hgt files) are for the Philippines?
    Thanks!

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  2. hi! just input the boundary extent (latitude , longitude) of the area you are looking for when you query for download at http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp

    ReplyDelete