| Re: Finding coordinates on the circumference of a circle [message #44819] |
Tue, 19 July 2005 15:21 |
James Kuyper
Messages: 425
Registered: March 2000
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Senior Member |
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guillaume.drolet.1@ulaval.ca wrote:
> Thanks a lot for helping me with this.
>
> This last suggestion with MAP_PROJ is great, although I don't quite
> understand everything. For example, do you specify the radius in meters
> because the Azimuthal Equidistant projection is in meters?
The default SPHERE_RADIUS is the true radius of the Earth in meters.
Therefore, radial distances in the Azimuthal Equidistant projection
will also be in meters. This depends upon knowing precisely how the map
projection works.
If you're not sure about the scale being used in a given projection,
select two points where you know:
a) what the true distance between those points is and
b) that the projection you're using preserves that distance.
Pass those points to MAP_PROJ_FORWARD, and from the results you can
calculate the scale factor being used, rather than guessing what it is.
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| Re: Finding coordinates on the circumference of a circle [message #44824 is a reply to message #44822] |
Tue, 19 July 2005 10:10  |
James Kuyper
Messages: 425
Registered: March 2000
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Senior Member |
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guillaume.drolet.1@ulaval.ca wrote:
> Hello group-
>
> I am preparing flight lines over a CO2 flux tower in Canada. I need to
> find the geographic coordinates of several points located on the
> circumference of a circle defined around the tower. The points are
> equally spaced (10 degrees), starting at 0 degrees (north) and
> incrementing clockwise. I want to do this for circles of different
> radii (10 km, 13 km, etc.). I need these coordinates in either WGS84 or
> NAD83.
>
> I found 'compass.pro' and 'findrng.pro', routines that find ranges and
> azimuths for given points (lat/lon) around a center location. I would
> like the opposite: find locations around a central point from arrays of
> azimuths and ranges.
The approach that involves calling LL_ARC_DISTANCE has a number of
difficulties, including the fact that it can only calculate one
position at a time, and it's not clear what datum is being used. Here's
an approach that allows you to do an entire circle at one time, and to
specify the DATUM to be used. You have to use the azimuthal equidistant
projection for this to work properly:
tower_latitude=75.0
tower_longitude=-90.0
proj = MAP_PROJ_INIT('Azimuthal Equidistant',
CENTER_LONGITUDE=tower_longitude, $
CENTER_LATITUDE=tower_latitude, DATUM=8)
radius = 15000.0; 15 kilometers
angles = !DTOR*10.0*FINDGEN(36)
x = radius*COS(angles)
y = radius*SIN(angles)
circle = MAP_PROJ_INVERSE(x,y,MAP_STRUCTURE=proj)
I checked the reasonableness of the results using
MAP_SET,tower_latitude,tower_longitude,/AZIMUTHAL,/GRID,/HIR ES, $
/LABEL,SCALE=1e6
oplot,circle[0,*],circle[1,*],psym=2
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| Re: Finding coordinates on the circumference of a circle [message #44827 is a reply to message #44824] |
Tue, 19 July 2005 08:11 |
K. Bowman
Messages: 330
Registered: May 2000
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Senior Member |
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In article <1121784988.644724.164530@g44g2000cwa.googlegroups.com>,
guillaume.drolet.1@ulaval.ca wrote:
> Hello group-
>
> I am preparing flight lines over a CO2 flux tower in Canada. I need to
> find the geographic coordinates of several points located on the
> circumference of a circle defined around the tower. The points are
> equally spaced (10 degrees), starting at 0 degrees (north) and
> incrementing clockwise. I want to do this for circles of different
> radii (10 km, 13 km, etc.). I need these coordinates in either WGS84 or
> NAD83.
>
> I found 'compass.pro' and 'findrng.pro', routines that find ranges and
> azimuths for given points (lat/lon) around a center location. I would
> like the opposite: find locations around a central point from arrays of
> azimuths and ranges.
>
> Does anybody know something that could help me?
>
> Cheers!
>
> Guillaume
The LL_ARC_DISTANCE function will return the lat and lon of a point at a given
azimuth and range from another point, although it doesn't know anything about
the datum. For short distances, and considering the precision of an aircraft
flight track, that may be adequate.
Ken Bowman
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