Details Ticket 3145


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Serial Number 3145
Subject R.profile does not use great circle paths
Area grass6
Queue grass
Requestors glynedwards@fastmail.fm
Owner none
Status open
Last User Contact Wed Apr 6 01:47:28 2005 (3 yr ago)
Current Priority 30
Final Priority 70
Due No date assigned
Last Action Tue May 3 11:20:59 2005 (3 yr ago)
Created Tue Apr 5 20:46:40 2005 (3 yr ago)

Transaction History Ticket 3145


Tue, Apr 5 2005 20:46:40    Request created by guest  
Subject: R.profile does not use great circle paths

Platform: GNU/Linux/i386
grass obtained from: Other (CDROM etc)
grass binary for platform: Downloaded precompiled Binaries

The r.profile command takes profiles along straight lines which is not correct
if the surface the profile is being taken over is the projection of the earth's
surface. In this case great circle paths should be taken.
 
The simplest fix is to update the man page so it is clear that the angle between
the starting and endpoints as if they were on a flat plane is used to calculate
the intermediate points (At least this is what it looks like from the quick glance
at the source code I took after I didn't understand the results I was getting).
If a great circle was used then this azimuthal angle/bearing would change as
you move along the path.

Hope this helps

Glyn Edwards
Wed, Apr 6 2005 01:47:28    Mail sent by hamish_nospam@yahoo.com  
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From Hamish <hamish_nospam@yahoo.com>
To Request Tracker <grass-bugs@intevation.de>
Cc grass5@grass.itc.it
Subject Re: [GRASS5] [bug #3145] (grass) R.profile does not use great circle paths
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> this bug's URL: http://intevation.de/rt/webrt?serial_num=3145
> ---------------------------------------------------------------------
> 
> Subject: R.profile does not use great circle paths
> 
> Platform: GNU/Linux/i386
> grass obtained from: Other (CDROM etc)
> grass binary for platform: Downloaded precompiled Binaries
> 
> The r.profile command takes profiles along straight lines which is not
> correct if the surface the profile is being taken over is the
> projection of the earth's surface. In this case great circle paths
> should be taken.
>  
> The simplest fix is to update the man page so it is clear that the
> angle between the starting and endpoints as if they were on a flat
> plane is used to calculate the intermediate points (At least this is
> what it looks like from the quick glance at the source code I took
> after I didn't understand the results I was getting). If a great
> circle was used then this azimuthal angle/bearing would change as you
> move along the path.
> 
> Hope this helps

are you working in a lat/lon projection?


see also d.geodesic

Description:
 Displays a geodesic line, tracing the shortest distance between two
 geographic points along a great circle, in a longitude/latitude data
 set.

and d.rhumbline too.

great circle path should be an optional flag to r.profile,r.transect I
guess, or maybe automatic when PROJ=LL.



Hamish


Tue, May 3 2005 11:20:59    Mail sent by glynedwards@fastmail.fm  
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From "Glyn Edwards" <glynedwards@fastmail.fm>
To "Hamish via RT" <grass-bugs@intevation.de>
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Subject Re: [bug #3145] (grass) R.profile does not use great
Date Tue, 03 May 2005 02:20:55 -0700
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> 
> are you working in a lat/lon projection?

Looks like I was using a Lat/lon projection, see below (This is gtopo30
data)

GRASS 6.0.0 > projection: 3 (Latitude-Longitude)
zone:       0
datum:      ** unknown (default: WGS84) **
ellipsoid:  sphere
north:      90N
south:      40N
west:       100E
east:       140E
nsres:      0:00:30
ewres:      0:00:30
rows:       6000
cols:       4800

Primarily I wanted to use r.profile in a script to take transects of
topography at right angles to a given great circle line. Unfortunately
because the r.profile lines are not along great circles the sample
points along the line didn't match up which alerted me to the problem. I
have since solved this by using GMT but thought I'd better file a bug in
case anyone else used r.profile in the wrong situation. It'll still be
fine for small angular separation

> 
> see also d.geodesic
> 
> Description:
>  Displays a geodesic line, tracing the shortest distance between two
>  geographic points along a great circle, in a longitude/latitude data
>  set.
> 
> and d.rhumbline too.
> 
> great circle path should be an optional flag to r.profile,r.transect I
> guess, or maybe automatic when PROJ=LL.
> 
Sounds good, and I guess the code for finding the points along the line
are already
in the d.geodesic program. I'd try and code a version myself but I have
never coded in c and unfortunately I'm trying to write up a PhD at the
moment :-).

Thanks 
Glyn

-- 
  Glyn Edwards
  glynedwards@fastmail.fm


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