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Mar
8
comment CRS and QGIS georeferencing for Cassini projection
@Andrej. Very probably!
Mar
8
comment CRS and QGIS georeferencing for Cassini projection
Thanks, AndreJ. I'm slowly getting the hang of it all ;)
Mar
8
accepted CRS and QGIS georeferencing for Cassini projection
Mar
8
awarded  Supporter
Mar
7
awarded  Editor
Mar
7
revised CRS and QGIS georeferencing for Cassini projection
Additions as requested as per comments. And the proj4 string formatted as code.
Mar
7
comment CRS and QGIS georeferencing for Cassini projection
@AndreJ My understanding is that Cassini assumed a sphere although the ability to work with ellipsoids came later but I have tried this with airy which, if an ellipsoid was in use c1850, would have been the one in use. Will add the points file to the question.
Mar
6
asked CRS and QGIS georeferencing for Cassini projection
Jun
27
comment Make a polygon from intersecting lines in QGIS, using Polygonize or similar?
2.4 is now out. I just installed it and tried polygonize on a small test set. It still throws the divide by zero error.
Jun
27
comment Make a polygon from intersecting lines in QGIS, using Polygonize or similar?
2.4 is now out. I just installed it and tried polygonize on a small test set. It still throws the divide by zero error.
Oct
7
comment How to start from scanned maps in qgis
After some discussion in chat & a good deal of experimentaion with different versions there appears to be a problem with calculating coordinates. 1.4.0, 1.7.0 & master are all OK under Debian Squeeze as is 1.8.0 on Windows but 1.8.0 on Debian gives results a hundred metres or so off. Bug #6472 has been logged.
Oct
6
awarded  Commentator
Oct
6
comment How to start from scanned maps in qgis
[link](streetmap.co.uk/… &ar=y&mapp=map.srf&searchp=ids.srf&dn=585&ax=413295&ay=405983&lm=0) is the point. OS Opendata vector roads agrees with SM's coordinates. QGIS, using the CRS calculator above or OTF reprojection from Openlayers is consistent with itself. OS is also consistent between coordinates on SM & downloaded vector files but differ from the conversions from lat/long. If QGIS/GDAL & OS are both nominally using EPSG:27700 their algorithms differ. Who's right?
Oct
6
comment How to start from scanned maps in qgis
I ran out of characters on the previous comment & should have added that I used on-the-fly reprojection with OpenLayers to use EPSG:27700. The following sets of coordinates refer to the same point in the middle of the sheet: 41390.5,406016.2; 413291.5,405989.5; 413190.5,406014.3; 413192.4,40613.7. The 1st & 3rd are georefed with Google Maps guess-the-centre-of-the-window & Streetmap lat/long converted by your parameters; the 2nd Streetmap's coordinates & the last OpenLayers OTF reprojection. SM's own coordinates are out of kilter.
Oct
5
accepted How to start from scanned maps in qgis
Oct
5
comment How to start from scanned maps in qgis
@Mapperz. The wikipedia entry you cite states the grid coordinate system was introduced after the retriangulation of 1936–1962. These are maps dating from a century earlier.
Oct
5
comment How to start from scanned maps in qgis
@Mapperz. This is a significant issue. As one of the objectives is to track changes in features one has to be very careful not to latch onto features which have changed. And a lot have in a century & a half.
Oct
5
comment How to start from scanned maps in qgis
The next attempts used Google Maps as suggested above & Streetmap. Streetmap provides cross-hairs which are easier to position but at higher zoom drops down to the OS vector material which has less choice of targets. They gave more or less the same lat/long results allowing for positioning errors. But SM also gives 6-digit OS Grid coordinates which are different from those given by the calculator. Clearly they use different parameters. The final approach was to use the OpenLayers plugin with Google as the canvas & use the coordinate from canvas method of the georeferencer. It's much easier.
Oct
5
comment How to start from scanned maps in qgis
Thanks @nhopton. It's been an interesting experience. I made several passes at this. The first was to use the converter above to use the lat/long for the corners as printed. The rationale is that it gives the longest baseline even if the map has no useful features in the corner, especially if the corner falls into a different county & the map is blank. The output was somewhat rotated & distorted & the coordinates of the corners were changed. This seems to indicate that the original projection was some way off EPSG:27700.
Oct
3
asked How to start from scanned maps in qgis