extend r.slope.aspect to compute the values at the edges

[hellik]

taken from the ML

​http://lists.osgeo.org/pipermail/grass-dev/2015-January/072779.html

Helena Mitasova - Sun Jan 4 06:36:27 PST 2015

Another way would be to modify r.slope.aspect to compute the values at the edges - a second order polynomial min.square approximation is used to estimate the derivatives in r.slope.aspect (you get the well known differencing function when you do the math) and it can be used also to compute the values at the edge cells, but I had no luck convincing others that it is the right thing to do.

I believe that any reasonable estimate is better than the current shrinking region (in r.flow we just propagate the same values to the edges), but that does not seem to be the consensus.

Also, implementation for the edges is not straightforward because of how GRASS works with rows, but a smart developer could certainly do it.

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It would be nice that r.slope.aspect compute the values at the edges, at least as an option. I agree that any reasonable estimate is better than the current shrinking region.

[neteler]

Milestone renamed

[neteler]

Ticket retargeted after milestone closed

[martinl]

All enhancement tickets should be assigned to 7.4 milestone.

[neteler]

Ticket retargeted after milestone closed

[mmetz]

Implemented in r72653 with a new -e flag to compute values at edges, idea and method from gdaldem -compute_edges.

[martinl]

All enhancement tickets should be assigned to 7.6 milestone.

[martinl]

Ticket retargeted after milestone closed

[martinl]

Ticket retargeted after milestone closed

[veroandreo]

Replying to mmetz:

Implemented in r72653 with a new -e flag to compute values at edges, idea and method from gdaldem -compute_edges.

I just discovered the issue of null borders in the output of G7:r.slope.aspect and I tested this flag in grass dev, but nothing changes. Is there anything else that needs to be set?

This is what I do:

g.region vector=firestations align=elev_state_500m
r.slope.aspect -e elevation=elev_state_500m slope=slope_500 aspect=aspect_500

The output still has one pixel less all over the 4 borders. See the attachment.

[veroandreo]

output of r.slope.aspect using -e flag which is supposed to fix the issue of a missing pixel line in all edges

[mmetz]

Replying to veroandreo:

I just discovered the issue of null borders in the output of G7:r.slope.aspect and I tested this flag in grass dev, but nothing changes. Is there anything else that needs to be set?

You discovered a bug regarding the -e flag, fixed in master 1e46d88 and relbr78 1496ee1.

[veroandreo]

thanks! looks good now :) Closing this one as fixed.