GPS coordinates conversion: practical use in Spain, SigPac

Without my mediatek GPS working properly, I have found in my SD card several screenshots of GPS coordinates: several months ago I was in a house in the country and I was asked to use my GPS to verify if the boundaries where ok, as the owner was suspecting the adjoining property have moved several meters the boundary ... in his own benefit.

I didn't have any special GPS application installed in bada, so I just take some screenshots, waiting to the GPS accuracy indication was 0 m.
The GPS provided data with latitude and longitude, with decimals, as well as altitude.
In Spain, there is a web application to see land distribution, and it has HTML5 interface, SigPac 
http://sigpac.magrama.es/fega/h5visor/

There are several interesting options:
- To see as a vectorial layer "parcelas" (plots) boundaries
- To search for a location: "Consultas > Buscar > coordenadas", where just UTM coordinates are allowed.
There is a "Radio" (radius) option, with 10000 as default value: it is zoom dependent. With a 100 value the zoom value is 18 (maximum is 20). 
- Query a point's coordinates: "Consultas > coordenadas del cursor"


With GPS captured coordinates points , I convert them to UTM:
http://boulter.com/gps/
http://home.hiwaay.net/~taylorc/toolbox/geography/geoutm.html
I take a plot's screenshot and with Inkscape, I draw over it the points with the captured coordinates (Sigpac shows on the left bottom corner the coordinates, and I draw "approximately" in Inkscape the corresponding point in the screenshot)

With that, I have a new boundary line ... and it seems the neighbour has moved in some points almost 3 meters the boundary ...