Random Coordinates in Polygon

[Nicolas Orgeira]

Hi,

Using a shape file diving a city into 20 zones, I would like to randomly generate a GPS coordinate and test whether it is in a given zone, and keep on resampling until I have sampled 30 GPS coordinates in a given zone while ensuring that two GPS coordinates are no more than 50 meters away from each other. I think the steps to follow are:

1) Random select a GPS coordinate (using the shape file to bound the latitudes and longitudes per zone, or else it could take a while to get a first GPS coordinate in a given zone).
2) Randomly select a GPS coordinate within a 50 meter radius and check that this point is still in the same zone.

I was thinking of using gpsbound but that would only help to check whether the gps coordinate is in the correct zone. I would first need to generate the random GPS coordinate falling within the boundaries of the zone (polygon).

Any help/tips would be much appreciated!

[Scott Merryman]

Here is a start that makes use of Richard Picard's -geodist- and -geoinpoly- :

cd "C:\Users\Scott\Desktop\tmp"
clear
//shapefile
input   _ID _X _Y
1         .         .                                                       
1        -110        37          
1        -110        41         
1        -106        41        
1        -106        37          
1        -110         37         
2         .         .          
2        -106       37         
2        -106       41        
2        -102       41        
2        -102       37         
2        -106       37          
end

sort _ID
save coordinates,replace

//random coordinates
clear
set obs 10
set seed 2439
gen double lat = 37 + (41 - 37) * uniform()
gen double lon = -110 + (110 - 102) * uniform()
sum lat lon 

//determine in which polygon the random point are located
geoinpoly lat lon using coordinates, noprojection

//calculate distance from first point
sort _ID lat
by _ID: gen j = _n
reshape wide lat lon, i(_ID) j(j)

forv i = 2/5{
    geodist lat1 lon1 lat`i' lon`i', gen(distance`i')
}

reshape long lat lon distance, i(_ID) j(order)
l