Spanish to English Tech/Engineering Surveying

This is one of the services offered by a company that helps clients set up agricultural operations in Romania. I've put my provisional translation below. I don't have much knowledge or experience in mapping, and I'm a little unsure how this should all fit together, particularly in relation to the terms posted. TIA

Realización de mapa de la finca agrícola o forestal sobre una base de ortofotomapa a escala conocida y referenciada con coordenadas geográficas de localización, identificadas con GPS sobre el terreno o en base al plano catastral.

Producing maps of farms and forest holdings ***on the basis*** of an orthophotomap at a known scale and referenced with geographical position coordinates, identified with GPS on the ground or ***on the basis*** of the cadastral map.