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Least Cost Path - for dummies

JaredReid1

I have never used Least Cost Path before and the online help is WAY over my head.

Can someone please break it down for me how to find the Least Cost Path between 2 points?

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The data I have to weigh this path is as follows:

One NED from USGS with elevation in meters.
2 points (My start and end points)
4 High Consquence areas polygons (ecological sensitive, drinking water, other populated, and highly populated)

thanks,
Jared

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JaredReid1

Ok seems like I'm not getting much help so I'm going to post my trials and errors here.

Looks like my first step is to take my very large(geographically) DEM and clip it to just the area I want.   So I created a polygon over the parts of the DEM that I want to clip.   Then I went to the Toolbox - Data Management Tools - Raster - Raster Processing - Clip.

I then found out I need to reclassify my newly clipped DEM. (why I'm not sure) I turned Spatial Analyst extension on. Then selected my clipped DEM in the drop down box. Then clicked Spatial Analyst - Reclassify.   I choose to have 8 unique values based on the range of elevation. Those 8 values I assume I will be given weighted values later on. ie. 1-4 no cost, 4-6 medium cost, 7 high and 8 avoid.

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That now leaves me with my shapefiles (Which I want to weigh differently) that I guess I now need to reclassify.   I assume I will have to turn these polygons and points into rasters as well...

JaredReid1

I have now converted my polygons into rasters and then reclassified them.

My four layers are now reclassified with the values of "4" (4 will be my thou shall not pass number)

I then reclassified my reclassified DEM with the values of 1,2,3 and 4. (4 being the high elevation to avoid)

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I then read that you need to merge these together by using the raster calculator and adding these values together.

When I do, it generates a calculation raster with a High: 2147483647 and a low of -2147483648!

What happened to my neat and tight 1,2,3,4 values??

MarkBoucher

Looks like my first step is to take my very large(geographically) DEM and clip it to just the area I want.

You can also limit the area your will process using the Geoprocessing>Environments...>Processing Extents and set the processing extents to equal some feature class that just covers you project area.

MarkBoucher

I then read that you need to merge these together by using the raster calculator and adding these values together.

Have you tried the Data Management Tools.tbx\Raster\Raster Dataset\Mosaic command? This doesn't add them, it has several options such as use the value of the last raster listed. This will leave your values alone.

JaredReid1

Have you tried the Data Management Tools.tbx\Raster\Raster Dataset\Mosaic command? This doesn't add them, it has several options such as use the value of the last raster listed. This will leave your values alone.

Thank you! That solves that portion of my question.

Robert_LeClair

Jared -

One way to learn more about the least cost path workflow is to register for the Distance Analysis using ArcGIS 10 web course. It goes into straight line distance and least cost path analysis workflows that may assist you. I teach the now retired Spatial Analyst class for Esri - feel free to email at rleclair@esri.com if you have specific questions.

Regards,

Robert LeClair
Esri-Denver

JaredReid1

Jared -

One way to learn more about the least cost path workflow is to register for the Distance Analysis using ArcGIS 10 web course. It goes into straight line distance and least cost path analysis workflows that may assist you. I teach the now retired Spatial Analyst class for Esri - feel free to email at rleclair@esri.com if you have specific questions.

Regards,

Robert LeClair
Esri-Denver

Thanks for the reply Robert. Do they offer that course for Arcmap 9.3?

JaredReid1

Have you tried the Data Management Tools.tbx\Raster\Raster Dataset\Mosaic command? This doesn't add them, it has several options such as use the value of the last raster listed. This will leave your values alone.

Following up question.

Now that my rasters are merged.

How does my Cost Weight know that 4 is bad and 1 is good and not vice-versa?

Robert_LeClair

To my knowledge, the web course was written for ArcGIS 10.0 but the concepts still apply at 9.3.x.

With regard to 1=good and 4=bad, it's a human interpretation versus a computer interpretation. When you use the Weighted Overlay tool to create your cost surface, you have the option to create an evaluation scale of any form you wish. It's up to you to determine what's "bad" and what's "good." The key is to be consistent through the entire workflow. Make sense?

JaredReid1

That makes perfect sense. Thanks!

JaredReid1

1 bonus question for you guys.

I have a corridor or route that I want to show preferential treatment too. Is there anyway I can reclassify a raster and give it a negative value to subtract away from the cost of other rasters that may be on top of it?

Robert_LeClair

Okay, I'm assuming this is your workflow - please correct me if I'm wrong. You have x number of rasters to create your cost surface.

You used the RECLASSIFY GP tool to reclassify your rasters to the same "scale."
Once all your input rasters have been reclassified to the same scale, you used the WEIGHTED OVERLAY GP tool to create your cost surface.
Now you want the cost surface to have "good" cells dedicated to the preferred alternative.

So what I would try is the EXTRACT BY MASK GP tool with the original cost raster as the input, the route as the Input Raster or Feature Mask Data and create a new raster (RASTER1) that is for the route.

Then I would use the CON GP tool that uses an expression where VALUE IS NULL - would get a zero and non-NULLs would get a 1 or a 4 depending on what is considered "good" by you and create a new raster (RASTER2).

The last step is to use the CON GP tool again where:

Input Conditional Raster: RASTER2
Expression Optional: VALUE = 1 (or 4 - depends on what's good)
Input True Raster or Constant Value: 1 (or 4 - depends what's good)
Input False Raster or Constant Value (optional): Original Cost Surface Raster
Output Raster: RASTER3

So in SUM, what this "should" do is extract those raster cells that "overlap" with the preferred corridor. Then you set a condition where it's preferred cell value or NULL. Then you set a condition where if it's preferred cell value, the raster cell gets a 1 (or 4) and the other cells get the original cost cell value. "Should" work...good luck!

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