Problem with DEM and Path Distance

[MargaretBrown_Vega]

Hi all,

I am trying to calculate path distance, using a vertical factor table (created by Nico Tripcevich based on Tobler's hiking function, discussed here http://forums.esri.com/Thread.asp?c=93&f=995&t=138683, among other places).

I created a DEM from SRTM version 4. When I calculate path distance using the vertical factor table, then generate contours, I get fairly evenly-spaced lines that look more like Euclidean distances. This is mountainous terrain. One should not be able to travel as far in the same amount of time when climbing a mountain. In fact, Euclidean distance contours seem to be further from the source point, even in areas where there are mountains that I know take two to three hours to climb!

In the past I've calculated path distance before using another DEM (an earlier version of SRTM). To try and diagnose the problem I snagged this earlier DEM, and calculated path distance from the same point feature on both DEM's for comparison. My problem is best illustrated in the two attached graphics. In the first graphic, the red contour lines represent what was generated with the earlier DEM, and is what I expect. The blue contours are my problem: that is what is produced doing the exact same calculation with this new DEM I've created. I am going mad trying to figure out what the problem is with this second DEM.

I've been able to calculate slope with this newer DEM (see attached figure 2), as well as some other compound indices. But there appears to be something wrong with the DEM. Other analyses have looked okay, but the path distance does not. Just to illustrate the point further the image shows slope from the second, problematic, DEM, and the evenly spaced path distance contours (which make no sense given the terrain).

I compared the two DEM's, which are of the same geographic area, and there seems to be a big difference in elevation (600 m).

My plan is to go back and use the older DEM. But, I would like to know what the problem is with the newer DEM (assuming that's what it is). Any suggestions or feedback would be much, much appreciated!

Sincerely,
Margaret

[Anonymous User]

Original User: ph0b0z

So, i tried different extents, these are the resulting rasters:

493x345 works
657x460 doesn't work (which means, the circular pattern is displayed).

Anyone else still interested in solving the problem? 🙂

[Anonymous User]

Original User: CAUK

This is my problem as well, why hasn't there been any response from ESRI on this in over a year?

[MichalBirkenfeld]

Hi everyone!

I'm not sure if seeing such a long thread about the same problem I'm having makes me feel better or worse...
I'm trying to calculate path distance travel time, using a 5m cell size DEM as the surface raster, and a ASCII table with the vertical factor. I keep getting this Eucledean "spider web" type raster...

Any suggestions?

Has anyone been able to solve this issue?

It is driving me INSANE...

Thanks
Michal

[Anonymous User]

Original User: rwhitlow

Michal,

I'm about 90% positive that the problem is the path distance toolbar is unable to handle more than 250,000 cells in the raster analysis. The most effective workaround I've found so far is to either reduce your study area to a smaller size (a 500 x 500 cell square) or resample your raster to a larger cell size. Alternatively, if you are working in an area with relatively little elevation change you can omit the vertical raster from the analysis and use the cost surface and horizontal surface instead, or use the cost- tools rather than the path- tools.

Hope that helps.

Best,
Ray Whitlow

[MarkBoucher]

There is a way to increase the size of a raster ArcMap will deal with. This post describes the "AdvancedArcMapSettings.exe" registry editor to use to do that. http://forums.arcgis.com/threads/48666-ArcHydro-doesn-t-output-to-shapefile?p=203314#poststop

Also, I found under the menu "Customize>ArcMap Options>Raster Tab>Raster Dataset subtab - Max Number of Unique Values to Render" has the same number I put into the "AdvancedArcMapSettings.exe" interface. As a matter of fact, it appears to be the same variable and either interface changes the same value.

Maybe increasing this value will allow you to process a larger raster.

[Anonymous User]

Original User: rwhitlow

Mark,

Thank you! I just tried the ArcMap->Customize change with a larger raster and it calculated the path distance correctly.

[MargaretBrown_Vega]

Hi All,

I returned to this problem after a fairly long hiatus. I think I successfully ran the path distance analysis after making two changes. I converted my DEM to a 32-bit floating point raster (this alone did not produce a good result). Then, I followed Mark's suggestion. I went into the Customize menu and changed the Max Number of Unique Values to Render. The value was set to 65536. I wasn't sure what to change it to, so I just opted for 200000. I've attached an image which shows travel time in hours away from major rivers (white) using Euclidean distance (red contour), and travel time in hours using a vertical factor table (yellow contour). No more concentric spider-web lines.
[ATTACH=CONFIG]20501[/ATTACH]
However, I do believe the result still underestimates travel time (perhaps another issue).

Just thought I'd chime in with Ray - Mark's solution appears to work.

Best,
Margaret

[Anonymous User]

Original User: emarsh

I have been following this post since the beginning, with the same problem (and tears). I was enthused by the recent posts by Mark, Raymond, and Margaret. HOWEVER, once again, I have been let down (such a familiar feeling with Arc). I continue the plea for help. Even after adjusting the Max Number of Unique Values to Render, I was back to square one, with precisely problem (and settings) described by Margaret and Steve in their intitial posts. I get good results with a very small extent, making me think I have eliminated the myriad of other possible problems. My DEM is a 32 bit floating point GRID and my Max Number of Unique Values to Render is 10 million (just to be on the safe side).

One minor point that may be helpful. As described in Mark's post, the Max Number of Unique Values to Render can be adjusted two ways: In ArcMap (Customize>ArcMap Options>Raster Tab>Raster Dataset subtab) or in AdvancedArcMapSettings.exe (Raster tab). It took me a while to realize that my changes to this number within ArcMap were not taking effect. The new number I entered remained, even when I closed and re-opened the program.

However, when I opened AdvancedArcMapSettings.exe, the number was unchanged, set to the default (around 65000), and changing it had no effect. I had to open this program with Administrator Access (by right-clicking on the application, "Open as administrator", even though I was already logged in as the administrator), and only then did the change hold. Finally, the number was the same in both ArcMap and AdvancedArcMapSettings, as Mark points out it should be.

Much to my dismay, this did not solve the issue.

As I said, I am back to square one.
I am open to any ideas! Thanks for all the hints and input up to now.
Erik

[MarkBoucher]

Sorry to hear you are having problems. One other thing I've done that seem to have greatly reduced the crashed (and tears) is to copy the project to my local PC and not have it on the network. My suspicion is that using Spatial Analysis and geoprocessing over a network takes a lot more time and introduces more chances of errors as the data goes back and forth through the routers. I can't verify this since my understanding of networks and routers is limited.

So, if you are trying to work on rasters and datasets over a network, copy them all to a folder on your PC and try to make it work then. I can guarantee things will go much faster and can almost guarantee you'll have fewer processing crashes.

An easy want to do this, if the files are all in one folder on the network, is to choose the option under the "File" menu for the mxd properties and make the references to layers "relative" (I think. I can't check now b/c my Arc Map is busy). Then after you copy it all to your PC you don't have to remap the layer sources.

[Anonymous User]

Original User: sesary

Hi. I have been facing the same problem for some time, and in my case it happened to be an issue with the 10.1 version. Solved after installing Service Pack 1.
Hope this helps
Cheers

César Parcero-Oubiña