| Re: Azimuth and Offset XYZ position correction [message #48022 is a reply to message #48020] |
Thu, 23 March 2006 16:42   |
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Dave[2]
Messages: 6
Registered: June 2005
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Junior Member |
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Oops, your right I forgot the angle, we do have that information as an
output. The scanner we use collects data at 250 measurements/second and
scanes 270 degrees vertical and 360 horizontal.
Hypothetically speaking, if I was to feed ~5 million data points into
the methodology you described, what are the chances of it choking? Or
speed issues?
All in all though....great approach!!!
Dave
kuyper@wizard.net wrote:
> Dave wrote:
>> Hi All,
>>
>> Quick question for anyone out there. I am looking to write a script and
>> I'm wondering if anyone out there has done it already. I have a known
>> precise position, say X and Y (UTM coordinate) in meters. From that
>> precise position I am using a laser range finder to calculate a
>> distance measurement from my original XY location. Along with this
>> distance measure I obtain the exact azimuth.
>>
>> What I would like to do is calculate the precise XY location at the end
>> of the laser measurement.
>
> You don't mention the elevation angle of the laser finder. Without that
> information, the best we can do is assume that it's 0. I would imagine
> that the distances you measure are much smaller than the radius of the
> earth, which would allow the use of certain approximations, but I'm
> going to use a method that will give accurate results for any distance
> measured along a great circle on the surface of the earth all the way
> up to half the circumference of the Earth. First, set up the utm map
> projection:
>
> utm = MAP_PROJ_INIT('UTM', CENTER_LONGITUDE=clon, $
> CENTER_LATITUDE=clat, ZONE=z);
>
> ;If you don't know what the zone is for your UTM projection, you
> probably aren't using
> ;one, and you can drop that argument. Next, convert your known position
> to longitude
> ;and latitude:
>
> lonlat = MAP_PROJ_INVERSE(known, MAP_STRUCTURE=utm);
>
> ;Next, set up an azimuthal equidistant projection centered at your
> known position:
>
> azeq = MAP_PROJ_INIT('Azimuthal Equidistant',
> CENTER_LONGITUDE=lonlat[0], $
> CENTER_LATITUDE=lonlat[1]);
>
> ;The next step depends upon how you define azimuth - there is a
> standard convention; ;unfortunately, there are many different mutually
> incompatible conventions. 0 degrees
> ;can represent either due North, or due East. A positive azimuth can
> represent either a
> ;clockwise or a counterclockwise rotation, as seen from above. I will
> assume that due
> ;North is 0 degrees, and that due East is +90 degrees. Then calculate
> appropriate
> ;offsets:
>
> dx = distance*sin(azimuth*!DTOR)
> dy = distance*cos(azimuth*!DTOR)
>
> ;Now convert those offsets to a latitude and a longitude.
>
> newlonlat = MAP_PROJ_INVERSE(dx, dy, MAP_STRUCTURE=azeq);
>
> ;And finally, back to UTM XY coordinates:
>
> newxy = MAP_PROJ_FORWARD(newlonlat, MAP_STRUCTURE=utm)
>
> There are other ways to do this, and some are more efficient, but this
> one hides all of the spherical trig inside the map projection code,
> allowing you to concentrate on other issues.
>
> There should be a standard function to do this; it's sort-of the
> inverse of map_2points(). However, I couldn't find one using the
> built-in help.
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