v.to.db azimuth direction

[davidinosauro]

Working for the first time with v.to.db and azimuth calculations, it took me a little while to figure out that angles are compute clockwise. I have no idea whether or not other GIS systems use this same convention, but in mathematics I have always used positive angles counter-clockwise.

I believe one line in the docs about this would not hurt and could improve a first-timer experience, even if this were a standard in GIS.

Davide

BTW, how is the doc-writing organized? Is there some pull-request system, so that I can patch it myself and let you just accept the changes?

[neteler]

Fully agreed.

For text snippets: If you could write up the change and add it here (ideally) as "svn diff", it would be most easy to integrate. Or the full file starting from the current HTML code and we diff ourselves.

[hamish]

v.to.db is a bit backward in this respect, elsewhere in grass the convention is always cartesian (theta CCW from the +x axis) not nautical (degrees CW from north). Without saying that one way is more appropriate than the other, I believe that for consistency v.to.db should be changed in trunk to match the rest.

I've been slowly adding options to modules to allow both, but default to cartesian:

      aspect_type   Direction map aspect type
                    options: cartesian,compass
                    default: cartesian

where cartesian = 90 - compass and vice versa. you can do that in db.execute to add the alternate convention into a new column.

and if v.to.db+azimuth doesn't already error-out in a lat/lon location, it should. (even if using geodesics, great circle azimuth has two diff't answers depending on which endpoint you use, and it transitions along the line)

patches for docs always welcome, Hamish

[neteler]

It seems that explanations have been added (long time ago):

• ​http://grass.osgeo.org/grass64/manuals/v.to.db.html
• ​http://grass.osgeo.org/grass70/manuals/v.to.db.html

Can the ticket be closed?