Determining whethera slope goes up or down...

[MichalBirkenfeld]

Hi,
I'm trying to create a cost surface for hill walking time expenditure, based on slope and distance.
To do so, I need to know whether at a specific cell the slope is positive (= walking uphill) or negative (= walking downhill).
Any ideas?
Thanks
Michal

[DanPatterson_Retired]

Perhaps Path Distance tools  http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Understanding_path_distance_analysis/0...

[MichalBirkenfeld]

Hi Dan, thanks for the quick reply...
I've tried using these, however, the "path distance" requires an existing cost surface.
As for the path distance allocation tool, I couldn't find where the actual factors should be entered (i.e. the conditions that transform a certain slope to time costs).
Any suggestions?
Michal

[DanPatterson_Retired]

Try searching PathDistance on this forum ie http://forums.arcgis.com/threads/10487-Pathdistance-problem?highlight=PathDistance

[MichalBirkenfeld]

You were right! Path distance is the way to go, and I've finally figured out how my data needs to be stored to use as input...
However, I've encountered a new issue - I keep getting a "spider-web" type raster, where the cost just  increases with distance, with no regards to the other factors.

I'm using a 5m cell size DEM as the surface raster, and a ASCII table with the vertical factor.

I found a very long thread, dealing with the exact same problem: 

http://forums.arcgis.com/threads/23888-Problem-with-DEM-and-Path-Distance?highlight=path+distance

I'm not sure whether this makes me feel better or worse, but I was wondering if you had any ideas??

Thanks
Michal

[RamB]

Hi,

I suggest you work with Aspect map (spatial analyst). For example let us say you are walking north, that is if you are on an average moving north compared to your starting point.

Step1. Develop an aspect map of your study area.
Step2. convert your walking path to a raster (path-raster), but of same size and extent as your study area. Such that cells beneath path are valued 1 and all others valued 0.
Step3. Multiply your path raster with your aspect raster and get the output.
Step4. In the output raster if the values are between 90 degree and 270 degree, then you are walking uphill. if the values are between 270 degree and 90 degree, then you are walking downhill. A negative value in your output suggests that you are walking on a flat surface.

This is because you are walking north and the degrees are measured from north.

If you are walking in any other direction, just play around with the degree ranges to get a solution. Let me know if you dont follow my english.

regards

[MichalBirkenfeld]

Hi,

I suggest you work with Aspect map (spatial analyst).

Hi Srirama,

Thank you for your reply!
That works well, especially when working with a specific path/direction.

I am trying to model catchment areas, or exploitation areas around archaeological sites, so my main interest is in defining walking time parameters  (for example, what area is controlled from the site, within 1 hour walking distance).

I think path distance tools are perfect for that, as they calculate the cost of moving through the terrain, while taking slope and direction into account. Unfortunately, I've encountered a major problem with the tool, which, according to the thread I found in the forum, has been driving people crazy for over a year now... The tool is basically calculating the cost according to distance, with no regards to the vertical factors I've put in. I'm working with a 5m cell size DEM as the surface raster and an ASCII table for the vertical factor.

Here is the thread on the issue http://forums.arcgis.com/threads/23888-Problem-with-DEM-and-Path-Distance?highlight=path+distance

any suggestions?

Thanks!
Michal

[RamB]

The tool is basically calculating the cost according to distance, with no regards to the vertical factors I've put in. I'm working with a 5m cell size DEM as the surface raster and an ASCII table for the vertical factor.

I understand you are going crazy 🙂 But if i am not asking for too much. Which vertical factors are you considering ? Please elaborate.
Thank you.

[MichalBirkenfeld]

I'm using an ASCII table that includes the slope (in degrees) and a factor (calculated according to hill walking reconstructions), which adds x amount of time cost for up hill walking or for descending down a very steep slope, and subtracts x amount for walking down a moderate slope.
It basically has two columns. one with the slope degrees starting with -90 and up to 90 and the other with the appropriate cost.

And yes - I am going slowly crazy over this 🙂 I've tried every idea that appeared on the older thread, including transforming the rasters to integers, the source file into a raster and back, etc. everything except magic 🙂

Thanks again
Michal

[RamB]

Hallo,

Finally I had time to work on this. I do not get euclidean shapes. I get proper anisotropic contours. Are you giving any cost raster as input ? I did not. I guess it get confused with cost raster if you use it. I just used DEM and a point file. It is a 32bit raster with 30x30 resolution. I changed to 5x5 resolution, that also works.

Did I get you right ?

regards,

PS: extent gives a problem I think