[GRASS-user] How to find out an angle between to points on the map (for r.plane, r.lake)

[Michael Barton]

On Mar 1, 2008, at 1:47 AM, Martin Landa wrote:

> Micheal,
>
> 2008/3/1, Michael Barton <michael.barton at asu.edu>:
>>> 2008/2/29, Dylan Beaudette <dylan.beaudette at gmail.com>:
>>> [snip]
>>>> Thanks for the update. I don't regularly use either of the GUI and
>>>> did
>>>>  not know about this feature. Does the patch that I submitted
>>>> interfere
>>>>  with this functionality? I think that it would be useful to have
>>>> this
>>>>  functionality in the old style monitors as well.
>>>
>>> d.measure is not used in wxGUI. Display windows are not  
>>> registered as
>>> GRASS displays (d.mon -L) [TODO, not sure how to implemented], I  
>>> think
>>> it would be good to use d.measure or something like g.measure in  
>>> wxGUI
>>> too instead of using Python-based function for distance calculation
>>> (at least it would make sense for LL projections)? Please correct me
>>> if I am wrong.
>>
> Michael,
>> What's wrong with the Python one?
>
> duplication of the code, and in this particular case ignoring geodetic
> distance for LL projections.

Well, my understanding of the direction of GRASS is that by v7, we'll  
lose all the interactive xterm-specific d.* modules. Interactive use  
will all be shifted to whatever GUI we design, rather than being hard- 
coded into a module. Simple, interactive measurement between 2 points  
on a display screen seems to me something to be handled by a GUI.

d.measure will go away OR change to a module in which you enter a  
start and end point and it returns the distance between the points.  
In the latter case, it could then be 'plugged into' a GUI as a  
function to calculate distance between points interactively 'grabbed'  
by the GUI.

Ignoring geodetic distance for LL projections is simply ignorance (on  
my part here) of the equations needed to do it. It's easy to  
determine if a location is LL. In that case, it should calculate  
geodetic distance rather than (or perhaps in addition to) displaying  
distance in map units. Just need to have the functions to do this  
added to the measurement methods.

Michael