Finding Latitude and Longitude of the 4 corners of a map using mapscript.

[Sean Gillies]

Troy,

I'm cc'ing the list on this.

First of all, you should provide a datum or ellipsoid, if known, for
the latlong projection.  The second thing to keep in mind is that your
projected rectangle's corner points are not necessarily the same points
a the original rectangle.  The projected rectangle is the smallest
rectangle that contains the transformation of the original one.

Sean

On Apr 29, 2005, at 5:50 PM, Troy R. Johnson wrote:

> Hi Sean, thanks for your information.  I followed the instructions and
> did this after I had drawn the image and saved the file:
>
> $image=$map->draw();
> $image_url=$image->saveWebImage('MS_PNG',1,1,0);
>
> Then I do a
>
> $latlong_rect = $map->extent;
> $latlong_rect->project( ms_newprojectionobj($map->getProjection()),
> ms_newprojectionobj("proj=latlong"));
>
> I then print these lat long values out, but they don't match up
> exactly with Lat Long
> points of fixed locations.  For example I have a shape file that
> contains a point at
> -111.383, 42.333.  When I pan that point to the corner 0,0 I get a
> reported lat lon
> of the corner printed out of -111.351, 42.344 and this is when I'm
> zoomed way in.
> The opposite corner lat lon points are -111.38 and 42.318.  So I'm
> surprised that there
> is this much error.
>
> Is there something else that I could be doing wrong?
>
>
> The reason for me wanting the lat, lon points is so that I can
> calculate the x,y
> pixel coordinates of these points of interest in shape files.  I'm
> sure the map
> server at some point knows what these x,y coordinates are, because it
> places those
> points on the image it returns, but I haven't been able to find a way
> to
> retrieve the x,y coordinates directly, so I'm calculating them based
> on lat, lon.
>
> Maybe there is an easier way to do this that I'm missing?
>
> Any help would be appreciated.
>
> Thanks
>
> Troy
>
>
> -----Original Message-----
> From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU]On
> Behalf Of Sean Gillies
> Sent: Tuesday, April 26, 2005 3:08 PM
> To: MAPSERVER-USERS at LISTS.UMN.EDU
> Subject: Re: [UMN_MAPSERVER-USERS] Finding Latitude and Longitude of
> the
> 4 corners of a map using mapscript.
>
>
> Just make sure to get and transform your extent *after* the map is
> drawn because msDrawMap will fudge the map extents to fit the specified
> image size.
>
> cheers,
> Sean
>
> On Apr 26, 2005, at 2:57 PM, James Sohr wrote:
>
>> Yes, you can calculate the lat long of your current extent by using
>> the
>> RectObj's project method. For instance:
>>
>>
>>    $latlong_rect = $map->extent;
>>    $latlong_rect->project( ms_newprojectionobj($map->getProjection),
>> ms_newprojectionobj("proj=latlong"));
>>
>>
>>
>> Assuming $map is your map object, the latitudes and longitudes of the
>> four
>> corners of your map are now stored in $latlong_rect->minx,
>> $latlong_rect-
>>> miny, $latlong_rec->maxx, and $latlong_rect->maxy .
>>
>>
--
Sean Gillies
sgillies at frii dot com
http://users.frii.com/sgillies