Re: [Talk-GB] man_made=survey_point

[Jass Kurn]

Gregrs has provided converted data for trig points, with the data obtained
from a FOI request. They created a page to explain the process, and made
available the converted data as a gpx file
https://wiki.openstreetmap.org/wiki/Ordnance_Survey_triangulation_stations .

Jass

On Sun, 23 Aug 2020 at 15:13, Nick  wrote:

> My thinking was that most people surveying would not use accurate and
> precise systems such as differential GPS and/or RTK. So if these systems
> were used to accurately and precisely locate distinct local markers (i.e.
> trig points, benchmarks etc.) then local surveys could potentially use
> these to refine/check their own surveys. This approach would still be based
> on community input but could be used as an approach to education (e.g.
> local schools involved) as to how surveying works in practice.
> On 23/08/2020 12:27, SK53 wrote:
>
> This approach has been advocated in other European countries, and the
> Spanish community imported all the points of the national geodesic network
> (e.g., for Extremadura
> ).
> They more or less violate the idea of OSM as something which is community
> contributed (IIRC each point has "DO NOT MOVE") and often interfere with
> objects which do need mapping (churches are a particular point). It's not
> clear that this import has assisted improved accuracy of mapping in Spain.
>
> Many trig pillars are now way out of alignment and mainly of interest as
> an artefact. Even benchmarks might not have much relevance as OS surveying
> mainly uses differential GPS with reference to their own base network (OS
> Net
> ).
> (From the OS website "Ordnance Survey (OS) benchmarks and their heights
> haven't been regularly maintained for over 40 years.").
>
> OS Net is effectively proprietary, there are a limited number of open base
> stations for differential GPS in the UK. I do believe differential GPS
> (RTK) has a role to play in OSM surveying, although for specific purposes
> rather than generic improvement of feature alignment.
>
> Regards,
>
> Jerry
>
> On Sun, 23 Aug 2020 at 10:05, Nick  wrote:
>
>> I have been looking at what is recorded under this tag in my area. I see
>> that there aren't that many and those that are on OSM refer to trig
>> points (see also http://trigpointing.uk/). My thinking is that if these
>> are accurate and precisely marked on OSM then perhaps they could be used
>> for resolving issue such as aerial imagery offsets.
>>
>> I therefore wondered if it was worth using other data under this tag -
>> specifically benchmarks (https://www.ordnancesurvey.co.uk/benchmarks/)
>> as there are huge numbers in the UK. If these were marked on OSM and
>> their accuracy and precision verified (OS open data is to the nearest
>> 10m square and transforming that adds errors), they could be helpful in
>> local surveys where they are less than accurate but also for ensuring
>> that moving all nodes in an area is valid (not just to match aerial
>> imagery). A possible linked organisation with data is
>> https://www.bench-marks.org.uk/
>>
>> Incidentally, the benchmarks can be helpful if you need to align
>> historical maps which have benchmarks shown.
>>
>> Any thoughts?
>>
>>
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Re: [Talk-GB] New Bing Imagery

[Jass Kurn]

On Wed, 19 Aug 2020 at 13:58, Colin Smale  wrote:

> Possibly even better that StreetView imagery is data that has been
> imported directly from OS, such as OS Boundary-Line for the admin
> boundaries. This is probably the closest we can get to cm-level accuracy -
> even though they don't give us the full resolution, the base points such as
> tripoints where boundaries meet are likely to be pretty damn accurate. I
> would recommend using these as a kind of calibration point to sanity-check
> imagery alignment and other data based on less accurate GPS positioning
> (e.g. from any consumer-grade GPS kit).
>
I've been coincidently wrestling with this issue of offsets for the last
two days. New Bing imagery is resulting in very detailed and useful mapping
(e.g. solar panels) but imagery is nearly always out by a problematic
amount. I also feel the best source for offsets is not Streetview, but the
vector data from OS OpenData. I've done some experimenting over the last
two days, and my favourite source for alignment is "OS OpenData Local -
Vector". Within those downloaded files is a data set called
"FunctionalSite" which is primarily the boundaries of educational sites.
They're excellent for alignment because the file is not too big, school
sites are common, and the boundaries are commonly thin fences which are
easy to align with Bing imagery, bringing errors to a trivial amount.

I think the secondary useful task is to make offsets available using the
"The Imagery Offset Database". It's been around for a long time, but is now
way more useful due to the issues being discussed.

With imagery likely to become more detailed, with more high quality tracing
of the high quality but misaligned imagery, I think we'll need a more
formal approach to "Align imagery before tracing guidelines"

Jass
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