distance measure

[piero cavalieri]

Thanks for your answer.
My distance measurement are very close (about 100 m) so the heart 
curvature has no influence.
Moreover, the raster dataset which is used for the measures (which has 
projected coordinates), is 1m resolution. So I think it's better for me 
to use trivial plane geometry.

Thanks again
Piero




Jeff Dege wrote:
> These linear formulas are only accurate at tight zooms - maps of 
> cities. When you're dealing with maps of countries or states, you're 
> going to have to use spherical geometry. The formulas for spherical 
> distance for coordinates expressed in lat/long are well known. But you 
> need to reverse the projection, converting the points to lat/long.
> // r and d in km. latitudes and longitudes in radians
> var r = 6371; // radius of the earth
> var d = Math.acos( Math.sin(lat1)*Math.sin(lat2) + 
> Math.cos(lat1)*Math.cos(lat2)*Math.cos(lon2-lon1) ) * r;
> Of course, if you want real precision, you'l want to use an ellipsoid, 
> instead of a sphere, but then you're getting into which ellipsoid is 
> defined for the datum you're using, and it starts to get hairy, again.
> Me, I'd run the numbers for the largest distance you're likely to 
> measure, once for linear distance and once for spherical distances, 
> and see how far apart they are. Then you can judge whether the 
> complexity of the spherical calculations is needed.
>
>     ------------------------------------------------------------------------
>     *From:* UMN MapServer Users List
>     [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] *On Behalf Of *Piero Cavalieri
>     *Sent:* Friday, May 04, 2007 10:07 AM
>     *To:* MAPSERVER-USERS at LISTS.UMN.EDU
>     *Subject:* [UMN_MAPSERVER-USERS] distance measure
>
>     Hi,
>
>     I use jBox to calcolate distance measure.
>     In jBox documentation I found this function:
>
>     function measure_handler(name, s, t, n, a) {
>     // c = 1 / 72dpi / 12inches_per_foot * 1.0003state_plane_scale_factor
>     var c = 0.0011577546296296
>     ;    // constant
>     var f = [scale] * c;           // scale factor
>     if ((s>0) || (t>0)) {
>     defaultStatus = "This segment = " + s*f + ", Total = " + t*f + ", Number of vertices = " + n ;
>
>     }
>
>     Since I'm not able to have user dpi (and 72 is a mean value, I
>     suppose), isn't more correct (and simpler) to use this way: ?
>
>     Distance = (x2 – x1)/ mapWidh * t
>
>     Where x2 and x1 are mapextents in real world coordinates, as
>     returned in imgext, mapWidth is the width in pixel, as specified
>     in .map file (and returned in template file), and t is the jBox
>     meaure, in pixel.
>
>     Thanks
>
>     Piero
>