In the context of last month's post regarding David Doyle's article, "Why Doesn't My Centimeter Match Your Centimeter?", guest columnist Denis Parrot expands on that issue by addressing the concept of the survey datum. While often overlooked by the novice - and sometimes by the professional - the datum that is referenced for any particular survey or collection of field data is critical for determining how well that data set will match up with any other survey.
For example, this image shows what can happen when map imagery is referenced to one datum while collected street data references a different datum.
As is evident, consistent use of the correct datum is critical for good results when combining data from various sources. Parrot goes on to describe differences between some datums or "reference systems". He emphasizes how important it is to answer the question, "But compared with what?" Data within any given survey set may be very precise, but how accurate is that survey when compared to another data set from another source? This applies to anyone using GNSS data, from the professional surveyor providing data to a client, to the farmer collecting his own topographic data, to the GIS user combining data from multiple sources.
For details, see the full GPS World article here: "Precise GNSS Positioning — but Compared with What?"