I have 50-meter DTM and 20-meter DSM data available to choose from to use for calculating a compound topographic index (CTI). While the 20-meter resolution is more attractive, logic tells me that when calculating the compound topographic index (CTI), one should be using a bare-earth elevation model (DTM), not a surface elevation model (DSM). If flow direction and accumulation are being calculated, using a surface model would not provide a good representation of earth surface hydrology/routing; however, I see many examples where SRTM or other DSMs are used as the input elevation surface. Is this a valid approach? Your comments and any references are appreciated.

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    This is a matter of inconsistent terminology. Just to cut to the chase, it is not correct to calculate CTI on a surface where heights above the ground are represented. This is a metric that should only be calculated on a bare-earth surface. The commonly available version of STRM is, in fact, a bare-earth DEM. You have to make a concerted effort to acquire the STRM data that is the true surface model that includes above ground objects. – Jeffrey Evans Jun 7 '13 at 18:51
  • To hopefully try and clarify the conflicting information regarding whether the SRTM elevation data set represents a bare-earth or surface model, I visited the CGIAR website, as suggested by @SoilSciGuy. I emailed Andrew Jarvis, one of the lead scientists who developed the "filled" SRTM elevation model. He says: "As far as I know, all SRTM data is a surface model, ours included!" – fbiles Jun 18 '13 at 22:07

A DSM may cause some problems when determining flow direction and subsequent flow accumulation and CTI/TWI. As you mentioned a DSM is a surface model, which contains vegetation and building heights. A good discussion at this link What is the difference between DEM, DSM and DTM?

  • @Jeff Evans and SoilSciGuy - Thank you, you both answered my question. I had already seen the "...diff between DEM, DSM, DTM" post, which is one of the few refs I found on the topic. The answer to my question seemed obvious to me (i.e., only use a bare-earth), but I was looking for confirmation. Thanks for the SRTM clarification. I was under the mistaken impression that it was a surface model. – fbiles Jun 11 '13 at 1:07
  • @Jeffrey Evans -- Regarding the SRTM elevation model (the one that is commonly available for download), this information from the USGS FAQ pagelink confuses me: "The chart below shows differences between the NED and SRTM collections. The primary difference is the elevation from the NED is a bare ground reading whereas SRTM is canopy based. The chart below shows differences between the NED and SRTM collections." – fbiles Jun 11 '13 at 2:08
  • It can be confusing because I have seen SRTM referred to as "first return" which implies it is indeed a surface model. As Jeff said, you should make sure you're getting the bare Earth model as some groups have processed SRTM to represent bare earth. I believe the SRTM data provided by CGIAR is bare Earth, but you will need to check. – SoilSciGuy Jun 12 '13 at 15:43

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