Points adjustment on the map

[Luis Trevi ño]

Hi,

I'm developing a vehicule localization aplication using mapserver CGI with
postgis and I need to adjust my cartography with real data that comes from
a GPS installed on the vehicules.

My problem is that the points (vehicules position) displayed on the map
are 40-60 meters up north than they should be. My map output projection is
proj=longlat ellps=GRS80 datum=WGS84 no_defs (the same as my cartography
data). The data from the vehicules are stored on a postgis table and has
this projection: proj=longlat ellps=WGS84 datum=WGS84 no_defs. (I've tried
using different kind of projection but I didn't get what I need).

this particular aplication requires some sort of precision, so I need to
adjust this difference and haven't found how.

Is there any way I can adjust this?

Thanks in advance,
Luis.