1 Reply by Jim Bacon

  • Jim Bacon
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  • From: BRC
  • Registered: 03-11-2008
  • Posts: 980

Re: Channel Island Grid References

Hi

I have been working on a module for Channel Island grid references which has caused me various headaches. This is a note of them.

There are four different grid reference systems that may be found on maps of the Channel Islands. There are the Guernsey Grid and the Jersey Grid, shown on new maps and now used by all official institutions on the islands. Also, on these maps, which I have not actually seen, there is, I am told, the UTM grid that gives us the WV/WA grid references that may appear familiar. However, this grid is based on the WGS84 datum which is different from the old maps which use the ED50 European Datum. There is a difference of about 200m between them. Biological records continue to be submitted against the old datum although paper maps showing this grid may no longer be in print.

I have endeavoured to support all of these systems in order to make Indicia attractive to local organisations in the islands who wish to use the new grids whilst supporting existing schemes that need to continue using the old grid. So far I have only been able to test against the old grid.

When testing I found my grid references did not come out quite right. After much head scratching I found this was due to the PostGIS settings for the old grid, known as projection EPSG:23030. Because there are multiple values for the European Datum the default is to do no datum transformation at all. I have applied the mean datum shift by updating the database as follows:

update spatial_ref_sys 
set proj4text = '+proj=utm +zone=30 +ellps=intl +units=m +no_defs +towgs84=-87,-98,-121'
where srid = 23030;

The datum values were obtained from http://earth-info.nga.mil/GandG/coordsys/onlinedatum/CountryEuropeTable.html

Jim Bacon

2 Reply by johnvanbreda

  • johnvanbreda
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  • From: UK
  • Registered: 27-04-2006
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Re: Channel Island Grid References

Thanks Jim, good work. Is the intention to put this script in to a db updates folder within the channel islands module, so it gets automatically applied?

John van Breda
Biodiverse IT

johnvanbreda's Website

3 Reply by Jim Bacon

  • Jim Bacon
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  • Registered: 03-11-2008
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Re: Channel Island Grid References

Hi

Just to further elaborate, I created a module, sref_channel_islands for the Guernsey and Jersey grids and an sref_utm for the utm grids. (and previously I added sref_osie for the Irish grid)

The utm module currently just works for locations in zone 30U but could be extended in the future.

I have put the database update below in to the module so that it is automatically applied. I am conscious that this is not a perfect solution as another country in zone 30 may wish to use Indicia with a different ED50 datum. At that point I think we create our own entires in the spatial_ref_sys table and allocate our own srid.

Jim