NASA have already sent several surface rover vehicles to the surface of planets, mostly to Mars. Just think about Curiosity.
Compared to the fact that these rovers are remotely controlled and have strongly limited energy resources (not so much they could bring with them, and have "only" solar panels to collect some), they can travel quite big distances. I heard that one of these Mars rovers (don't know which one?) could travel up to 20 kilometres on the surface of Mars.
However, is this enough to make general assumptions about the whole planet? I mean, even on Earth it matters, where those 20 kilometres would be; be it in the Amazonas, in the Himalayas, or in the New York city, etc.
Obviously, prior to organizing a landing project like the Curiosity rover, NASA have monitored the surface of Mars with additional devices — but is it accurate enough, coupled with data gathered from surface rovers?
If I wasn't clear, here's an example: Curiosity measures a temperature inside a crater, but this value may be different inside other craters and valleys.