Take my answers with a grain of salt. I'm no expert, but these are from my experience and current understanding.
On Sat, Sep 14, 2019 at 3:58 PM Edward Bainton <[email protected]> wrote: > A few questions: > - Over how wide an area does an offset obtained that way hold good? > This varies, areas with large elevation differences (for example) are much harder to truly align because the offset tends to have a much higher variance. Even outside of factors like elevation, sometimes imagery is consistently misaligned in one region, but inconsistently misaligned in another. I think the key thing is to just keep checking features against GPS; other imagery layers; and other features you know to be accurate. > - Are the old OS maps better/worse/same as this system? Are they an > alternative for zeroing imagery? > Again, no hard answer here. If there is a lot of GPS data available then I believe it's likely more accurate to use the average of that. However, in my experience the OS maps are at least consistently accurate enough that I would trust them if I didn't trust the GPS data in an area. > - If I know the grid reference of somewhere (eg, Environment Agency puts a > 10-fig reference on a plaque on their assets) is that any help? > Yes, this is one method of aligning a feature. JOSM editor (for example) allows you to create a node at specific coordinates and iD has a hotkey-openable pane which will show coordinates at the current cursor position. Keep in mind that the source of a feature's coordinates may not be 100% accurate, there's always a level of uncertainty and it's easier to decide the true position if that's known too. I believe there are also intricacies to be aware of when converting coordinate systems, but that's a little beyond my current knowledge.
_______________________________________________ Talk-GB mailing list [email protected] https://lists.openstreetmap.org/listinfo/talk-gb
