| 10 Nov 2011 08:08 AM |
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Hi everyone,
I have been automating the envi-topographic modelling function using the envi_doit, topo_doit script to calculate the slope from a DEM. but I found out that the topo_doit function cant handle DEM files in lat/long projection, because it gives an output image with weird values. the interesting thing is if I do it manually, through the toolbar.. it works perfectly (with the same input image in lat lon). Can anyone kindly give me an explanation about this? doesnt the envi toolbar - topographic modelling - slope function call the envi_doit, topo_doit function? or it does something else? is this a bug of the topo_doit function?
thank you very much.
Tania
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MariM
Veteran Member
Posts:2396  
| 10 Nov 2011 08:42 AM |
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I suspect the issue is that your DEM pixel size is in degrees (since it is geographic) but your elevation units are in meters. It states in the TOPO_DOIT documentation that "pixel size should be in the same units as the DEM" which means you must pass in the pixel size in meters rather than degrees in order to normalize the data so the slope and other parameters are meaningful.
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Deleted User
New Member
Posts: 
| 10 Nov 2011 09:25 AM |
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Hi MariM,
Thank you very much for your quick response. you are very correct that the input DEM is in degrees, but I specified the output also in degrees (I took the projection info and the pixel size from the input file). So you suggest me to change the projection units from degrees into meters? is there an envi function which does this? But the funny thing is that you dont have to specify this when using the topographic modelling function through the toolbar, does this means that the topographic modelling function translates the pixel size automatically when it notices it is in degrees?
thank you again!
Tania
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MariM
Veteran Member
Posts:2396
| 10 Nov 2011 02:24 PM |
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One easy way to see how your pixel size in degrees converts to meters is to use the Convert Map Projection tool in ENVI. Here you can select your DEM in geographic and in the convert projection parameters, change the projection to UTM. ENVI will automatically calculate the new pixel size in meters. Then you can just set up your pixel size parameter to be this value. Unfortunately, there is no exposed routine (or DOIT) to let you do this in ENVI.
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