[Proj] How to obtain (lat, lon ) of a point if I know a reference point and relative distance (km)?
Gerald I. Evenden
gerald.evenden at verizon.net
Mon Aug 7 10:46:46 EDT 2006
On Monday 07 August 2006 12:25 am, Zhuanshi He wrote:
> Dear All,
>
> I want to obtain (lat, lon) of a serials of points saved in a data
> file. Before do that, I need to know how can I do it for one point.
>
> The pionts came from Mecartor projection. Now, I have a reference
> point (reflat, reflon), and the distance (kilometers) (x,y)
> corresponding to same (reflat,reflon). How to calculate the unkowns
> (?lat.?lon)? Can proj do it?
>
>
>
> ---(Reflat,reflon)-----------------(x)---------
>
>
>
>
>
> --------- (y)----------------- ?(lat,lon)---------
Unfortunately, [l]proj does not have a lat_0 option but this can be
compensated for in the following manner:
Assume the reference lat and lon are 45N and 160w respectively. First
compute the northing for the latitude 45n:
lproj +proj=merc +ellps=intl
0 45
which gives:
0.00 5591388.07
For the remainder of the points, the run line will look like:
lproj +proj=merc +ellps=intl +lon_0=160 +y_0=-5591388.07 -I
For some sample data points (note: y negative south of reference latitude):
0 0
10000 10000
10000 -10000
the results would be:
160dE 45dN
160d5'23.381"E 45d3'49.31"N
160d5'23.381"E 44d56'10.433"N
The above example is the same for program proj.
--
Jerry and the low-riders: Daisy Mae and Joshua
"Cogito cogito ergo cogito sum"
Ambrose Bierce, The Devil's Dictionary
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