[GRASS-dev] r.watershed at poles

GRASS GIS GRASS GIS developer
1

Dear GRASS developers,

Would you know of an easy way to make r.watershed work in latitude/longitude over the pole of a planet (in my case, over Antarctica) by specifying that the polar boundary in lat/lon is not a sink?

I would hope that if this is not a difficult change, this will allow drainage to properly continue over the pole instead of terminating at it.

Thank you in advance!

Andy

2

On Thu, Oct 1, 2015 at 7:10 AM, Andy Wickert <andrewwickert@gmail.com> wrote:

Dear GRASS developers,

Would you know of an easy way to make r.watershed work in latitude/longitude
over the pole of a planet (in my case, over Antarctica) by specifying that
the polar boundary in lat/lon is not a sink?

Wouldn't be a stereographic projection more appropriate?

https://grass.osgeo.org/uploads/images/Gallery/raster/polar_stereographic.png

Best
Markus

3

On Thu, Oct 1, 2015 at 3:52 PM, Markus Neteler <neteler@osgeo.org> wrote:

On Thu, Oct 1, 2015 at 7:10 AM, Andy Wickert <andrewwickert@gmail.com>
wrote:
> Dear GRASS developers,
>
> Would you know of an easy way to make r.watershed work in
latitude/longitude
> over the pole of a planet (in my case, over Antarctica) by specifying
that
> the polar boundary in lat/lon is not a sink?

Wouldn't be a stereographic projection more appropriate?

https://grass.osgeo.org/uploads/images/Gallery/raster/polar_stereographic.png

Best
Markus

Thanks for the suggestion. However, my goal is to run r.watershed across
the whole globe, so a stereographic projection would an ad-hoc solution.

(I have also had problems in the past projecting between polar
stereographic and lat/lon, but this is a secondary issue.)

Andy

4

Hi Andy,
running r.watershed for the whole globe is IMHO not necessary. Just
run r.watershed for each continent (except Europe/Asia) and isles
(greenland, UK, ...). Everything that is parted by an ocean or a
channel can be processed independently.

Best regards
Soeren

2015-10-02 0:27 GMT+02:00 Andy Wickert <andrewwickert@gmail.com>:

On Thu, Oct 1, 2015 at 3:52 PM, Markus Neteler <neteler@osgeo.org> wrote:

On Thu, Oct 1, 2015 at 7:10 AM, Andy Wickert <andrewwickert@gmail.com>
wrote:
> Dear GRASS developers,
>
> Would you know of an easy way to make r.watershed work in
> latitude/longitude
> over the pole of a planet (in my case, over Antarctica) by specifying
> that
> the polar boundary in lat/lon is not a sink?

Wouldn't be a stereographic projection more appropriate?

https://grass.osgeo.org/uploads/images/Gallery/raster/polar_stereographic.png

Best
Markus

Thanks for the suggestion. However, my goal is to run r.watershed across the
whole globe, so a stereographic projection would an ad-hoc solution.

(I have also had problems in the past projecting between polar stereographic
and lat/lon, but this is a secondary issue.)

Andy

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5

Hi Soeren and all,

I appreciate the opinion, and in fact have split continents in the past, but the project that I want to undertake requires many calculations with variable sea-levels and would be much easier to run on a full globe (albeit with null values in the ocean at each particular time-step).

That, and this unfortunately still doesn’t address doing Antarctica without reprojecting.

So really what I am saying is that I would like to hear ideas about the technical side of r.watershed (maybe I should just email Markus Metz directly) and how to change where it creates sinks. I know that it is able to go across the 180 meridian, so this boundary has been modified for lat/lon, and it therefore in my mind would be feasible to tell it that the north and south poles are not a sink for flow.

If there is really no way to do this with r.watershed, I will write code myself, but I am hoping that I can instead just improve r.watershed.

Thanks,

Andy

···

On Fri, Oct 2, 2015 at 2:04 AM, Sören Gebbert <soerengebbert@googlemail.com> wrote:

Hi Andy,
running r.watershed for the whole globe is IMHO not necessary. Just
run r.watershed for each continent (except Europe/Asia) and isles
(greenland, UK, …). Everything that is parted by an ocean or a
channel can be processed independently.

Best regards
Soeren

2015-10-02 0:27 GMT+02:00 Andy Wickert <andrewwickert@gmail.com>:

On Thu, Oct 1, 2015 at 3:52 PM, Markus Neteler <neteler@osgeo.org> wrote:

On Thu, Oct 1, 2015 at 7:10 AM, Andy Wickert <andrewwickert@gmail.com>
wrote:

Dear GRASS developers,

Would you know of an easy way to make r.watershed work in
latitude/longitude
over the pole of a planet (in my case, over Antarctica) by specifying
that
the polar boundary in lat/lon is not a sink?

Wouldn’t be a stereographic projection more appropriate?

https://grass.osgeo.org/uploads/images/Gallery/raster/polar_stereographic.png

Best
Markus

Thanks for the suggestion. However, my goal is to run r.watershed across the
whole globe, so a stereographic projection would an ad-hoc solution.

(I have also had problems in the past projecting between polar stereographic
and lat/lon, but this is a secondary issue.)

Andy

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