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Problems with cost path analysis - not shortest distance

Question asked by geonetadmin on Nov 22, 2011
Latest reply on Nov 23, 2011 by csny490
Original User: Jenn23

Hi everyone,

This is my first time using the distance tools, and I must be doing something wrong, because the output doesn't look as I expect it to. That said, I can't seem to figure out why. Here is what I'm doing:

Aim: To determine the shortest path from a nest to marine foraging locations for an individual bird. The bird is restricted to traveling along water only (i.e., freshwater and ocean).

How I'm doing it: After reading multiple entries in this and other forums, I decided the best way to do this was to use a cost distance approach:
- I created a raster where all water = 1 and where all land = NoData. This is my cost raster.  I have also ensured that all nests intersect a value cell on the raster. 
- I then used the above as input to the cost distance tool, with the nest site as the feature source data, creating cost distance and backlink rasters that I then used as input in the cost path tool, with foraging locations as the feature destination data. The output is supposed to be the least cost path from the nest to each foraging location; since all water cells have equal cost, this should thus represent the shortest path, right?

The problem: The paths that are being output are not the shortest - I can easily look at the map and find routes that are much shorter. I've attached an image to illustrate this. The path along the first inlet (north-south) looks okay, but after that, the paths are questionable, especially upon entering the bay where all the foraging locations are. I've also attached an image of the backlink raster, which looks very odd. I understand the information provided in the backlink, but what isn't making sense is the "next" cell it is directing to. Why is the raster so "patchy" (once out of the n-s inlet)?

Additional note: I'm actually performing this analysis using Model Builder, iterating through several nests, with different selections of foraging locations based on the nest value; however, I obtain the same output from both the model and from performing the analysis manually on a single nest, so the problem is not in the model.

Any help would be much appreciated. My apologies if anything above is not clear.