Building a real-world model from incomplete data

I am trying to build a Unity model of my local park (approx 750m x 500m) from topgraphical map images and actual lat/long/altitidue observations from a handheld GPS. The aim is to allow people to explore the park virtually and see various what-if options for improvements.

I've looked at the mapping data from the UK Ordanance Survey which offers DXF files with a 5 meter resolution but I was unable to import their sample file into Unity or Blender and the Autodesk FBX conversion utility didn't help.

I would like a resolution of around 1 meter, at least for key areas. My thought is to use a mesh (or meshes) with variable sized triangles based on the data I have which I could selectively break into smaller triangles using actual observations for key areas.

I'd welcome any suggestions and ideas about how (or how not to) do this within Unity or other software (within a limited budget!)