[Qgis-developer] QGIS inconsistency

[Bernd Vogelgesang]

Hi Steven,

just having a quick look at the data you provided:
- your Munros.shp doesn't have 27700 projection, but some user defined and  
only becomes visible when setting to 27700 manually.
- the geojson and the shape file have quite some offset. (75 whatsoevers,  
meteres, miles ??)

Apart from the fact that there might  be a bug, your data seems to me more  
buggy ;)
Besides that, i never dare to do any spatial analysis within epsg 4326.



Greetz
Bernd

Am 18.10.2013, 21:13 Uhr, schrieb Steven Horner <steven at stevenhorner.com>:

> Hello,
>
> I have been doing a project to calculate mountain viewsheds, basically
> where a mountain summits can be seen from.
>
> I have calculated the viewsheds and polygonized to Vector. So far so  
> good,
> but then I wanted to show which mountains could be seen from the summit  
> of
> other mountains I appear to have 2 problems.
>
> 1. I ran a spacial query to show how many mountains (Munros) are Within  
> my
> polgonized viewshed. This returns the Munros. Unfortunately I get a
> different number in QGIS 1.8 to QGIS 2.0.1, I don't know which is true
>
> 2. I had saved my Viewshed originally as a shapefile with a CRS of 27700
> (British National Grid), I saved a version as GeoJSON with CRS of 4326.  
> If
> I run the same spacial query on this file I get a different number of
> Munros again.
>
> So I now have 4 different figures from what I would have expected to be
> identical.
>
> Is data lost when a shapefile is saved as a GeoJSON file in a different  
> CRS?
> and which should I beleive 1.8 or 2.0.1 or neither?
>
> If anyone wants to test this you can download the files here:
> Viewshed  
> Shapefile<https://dl.dropboxusercontent.com/u/2657852/AmBodach_shp.zip>-
> CRS of 27700
> Viewshed  
> GeoJSON<https://dl.dropboxusercontent.com/u/2657852/AmBodach_geojson.zip>-
> Converted fromthe above but with CRS of 4326
> Munros Shapefile  
> <https://dl.dropboxusercontent.com/u/2657852/Munros.zip> -
> CRS of 27700
>
> Wish I had realised there was something wrong before I had calculated 200
> of these.
>
> Thanks,
> Steven


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Bernd Vogelgesang * Siedlerstrasse 2 * 91083 Baiersdorf * +49-9133-825374