[Talk-GB] Quarterly Project 2020Q1

[Robert Whittaker (OSM lists)]

On Thu, 7 Jan 2021 at 13:39, James Derrick <lists at jamesderrick.org> wrote:
> The challenge is imagery is often mis-aligned and offsets are always
> required across multiple imagery providers with the primary being
> downloaded GPX traces. Over Christmas I've armchair updated all of the
> construction=residential I've been cycle surveying over Summer and even
> with my own averaged GPSr traces, imagery offsets are a necessary PITA.

If we're thinking about a Quarterly Project involving lots of tracing,
it would be really useful to have a UK-wide tile/data layer that we
can use to ensure a common alignment of the imagery. The previous Bing
imagery was well aligned, at least in my local area, so this wasn't a
problem for me previously. The new Bing imagery seems to have
noticeable offsets, and those offsets change significantly from place
to place.

There was some discussion on this list a couple of months ago about
using the recently released Land Registry polygons as a reference to
align imagery to. See
https://lists.openstreetmap.org/pipermail/talk-gb/2020-October/025295.html
and the following posts in the thread. However, it turns out that the
precise transformations used to convert from OS National Grid to
WGS-84 etc. matter, and simplified versions often result in errors of
a few meters. This can be significant if you're worried about
alignment for tracing individual buildings. Rob Nickerson was looking
into the details of the transformations, with help from others, but
I'm not sure how far he got.

Robert.

-- 
Robert Whittaker