On Mon, Jul 26, 2010 at 9:38 AM, Bob Jolliffe <bobjolli...@gmail.com> wrote: > Hi Jason > > On 26 July 2010 04:49, Jason Pickering <jason.p.picker...@gmail.com> wrote: >> Hi Knut, >> >>> It may be that we want to use DHIS as both a repository with full >>> precision (though not ridiculously artifical ones like 15 decimal >>> lat/lon) and have a faster way of renderin. But for a repo, I think >>> something like PostGIS is in order. Or we could just store things as >>> GML... >> >> Well, this is really the issue. If DHIS is going to be a repository, >> any self-respecting GIS geek would not use it if the application >> clipped precision. Although a few meters is not significant in terms >> of rendering a map, it may cause havoc on certain datasets, >> particularly if there are topological relationships between different >> layers. If a facility is related topologically to a road network, and >> the point is shifted a few meters, this may result in disturbance of >> the topology between these layers, rendering DHIS useless as a >> repository. ogr2ogr is perfectly OK as long as we are not dealing with >> these types of layers, but as soon as we start to think about >> relationships to other layers, we need to be very careful about how >> the data is preprocessed. > > Would you suggest then that the best place to clip precision would be > when the data is retrieved from the database for the specific view/map > rendering, rather than prior to it being stored? > > This would render the current convenience of storing as a geojson > string redundant as we would need to process the string on checkout > anyway. > > Can anyone say what the precision is on the shapefiles prior to > ogr2ogr conversion ie. are we introducing a new level of precision > here or is that 15 digit precision the precision of the source > shapefiles?
Quoting myself: "Here is a comparison of what I get in GeoJSON vs GML (converting from the same shapefile): GeoJSON: 38.415412, 1.750212 GML: 38.415411724082148,1.750212388592194" Both using ogr2ogr. So 6 vs 15 decimals. Knut > Bob > >> >> >>> We should be very conscient of not pushing the new, very simple >>> solution too far, for more complex functionality we should rather >>> employ Geoserver and PostGIS - and I still think this is the best >>> solution for a national repository. Our new way of storing orgunit >>> boundaries is a very small subset of such a full blown GIS solution, >>> but has the advantage of being simple, lightweight and portable. >> >> Agreed on both points, namely that the solution is lightweight and >> aimed at thematic mapping but other solutions would be more >> appropriate for use as a repository of GIS data. >> >> Regards, >> Jason >> >> >> >> -- >> Jason P. Pickering >> email: jason.p.picker...@gmail.com >> tel:+17069260025 >> > -- Cheers, Knut Staring _______________________________________________ Mailing list: https://launchpad.net/~dhis2-devs Post to : dhis2-devs@lists.launchpad.net Unsubscribe : https://launchpad.net/~dhis2-devs More help : https://help.launchpad.net/ListHelp
