Interpretation of Corridor function: Differences between Vector and raster

Doranda

Hey,
I'm using the Graphab 3.0 version and want to create corridors in my network (based on a 10x10 m raster). For the corridor setting I select the same max distance as for the linkset and in case of the raster calculation i don't choose a specific metric, but a beta-parameter of 1 (as this is also a fixed setting for the vector-calculation). After i calculate both raster and vector with identical settings, both results show the same "expansion" outside of habitat patches. The vector result is presented through polygon boundaries, the raster through the pixel values.
I wonder, what defines the expansion of the polygon and assigned raster values >0? Moving in direct or diagonal lines from one habitat patch to another, finding possible paths, while staying inside the max dist threshold?
The raster has values assigned to each pixel, what is their meaning? I can't find an explanation, how to interpret the values apart from representing a gradient of probability of crossing two patches. (Manual p.21)
Also, why does the corridor raster assigns values to pixels inside the habitats?

I would be happy to hear from you!
Have a nice day
Frederik

gvuidel

Hi,
For vector output, each link of the graph produces a polygon representing the area that
can be traversed between the 2 patches habitat ie. the set of possible paths connecting the 2
patches and having a distance less than the max distance parameter.

For raster ouput, each pixel of the corridor area will be weighted by a metric (or the product of the capacity of the 2 patches if no metric selected) and a distance decay (negative exponential).
Since the same pixel can be assigned to several corridors, the maximum value is kept.

Inside patches, the max value of the corridor is used to maintain continuity. If you do not want to have values inside the habitats, you can mask the raster with habitat patches.
Best,
Gilles

Doranda

Hey Gilles,
Thank you for your answer.

i came across another question, which i could not solve yet.
When calculating a corridor, no matter if raster or vector output, the results always exclude links inside the set max distance. These links are on average 20-25m shorter than the set max distance, so they should be used for the calculation.
Increasing the max distance to include these links is not feasible as the width of the other already calculated corridors would increase too.
What could cause this problem?
As i mentioned in my first post, the habitat raster has a resolution of 10 x 10 m.

Best
Frederik

gvuidel

Hi,
Did you calculate corridors from euclidean linkset ?
If this is the case, it is normal because the distances are approximated in pixels and therefore do not accurately reflect the euclidean distance.
Best
Gilles