5th July 2014, 11:39 AM
Bear in mind the direction and magnitude of "error" of a single GPS reading on a stationary device (when not using corrections from a base station) "wanders" over time, so if you want to tie in to nearby fixtures you need to be quick about it! After all, you want to have the same error on your control points as on your captured features. But the principle is sound - the "error" will be the same over a wide area at any one moment in time. (All to do with which satellites are seen, and their true known positions...)
Using features from OS mapping as a control is itself inherently risky, because their lines are only accurate to a set tolerance based on map scale. Even if you get a co-ord down to the nearest mm when you interrogate with your GIS, the data may only be accurate to the nearest 500mm as captured. After all, it just has to look right when viewed at the chosen map scale. (Also, where things are so tightly packed that they turn into an inky blob, the OS tend to "ease" them a bit for visual clarity.) And that doesn't even take into account the correction factor between GPS and terrestrial map projections!
Best to just accept that data from a typical hand-held is within a few metres of truth, which is usually fine for fieldwalking, photo surveys, big landscape surveys, etc...
Using features from OS mapping as a control is itself inherently risky, because their lines are only accurate to a set tolerance based on map scale. Even if you get a co-ord down to the nearest mm when you interrogate with your GIS, the data may only be accurate to the nearest 500mm as captured. After all, it just has to look right when viewed at the chosen map scale. (Also, where things are so tightly packed that they turn into an inky blob, the OS tend to "ease" them a bit for visual clarity.) And that doesn't even take into account the correction factor between GPS and terrestrial map projections!
Best to just accept that data from a typical hand-held is within a few metres of truth, which is usually fine for fieldwalking, photo surveys, big landscape surveys, etc...