40 reputation
3
bio website
location
age
visits member for 2 years, 2 months
seen Jul 15 at 12:26

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
Oct
3
comment How to merge rasters with negative extents?
Thanks @Simbamangu. It didn't, in the end, get me to where I wanted. When I then try to overlay onto a scanned OS map however many or few GCPs I provide it puts it down about half a mile S of where it should be. I think I'm going to have several problems with this project & ISTM that this particular one was somewhere in the middle. I'll start a new question at the beginning & see how much else falls into place
Oct
3
awarded  Scholar
Oct
3
accepted How to merge rasters with negative extents?
Oct
2
awarded  Student
Oct
2
comment How to merge rasters with negative extents?
I tried another merge on the pair of images which gave be the black wedge above & this time ticked the No data box & the generated image is now OK. How do I update the title to say solved?
Oct
2
comment How to merge rasters with negative extents?
Thanks, @Simbamangu, I'm getting there. I built a points file to set three of the corners to what they should be, eg. top left, which was 0,0 becomes 0, 1651 & bottom left which was 0,-1651 becomes 0,0 & applied that as a linear transform. I then georeferenced the other image as before. This looked OK when displayed as layers with a spline transform but has a wedge-shaped gap when I generate the merge. So I'll go back & try your suggestion of georeferencing for the edges only.
Oct
2
comment How to merge rasters with negative extents?
Thanks, Simbamangu,
Oct
2
asked How to merge rasters with negative extents?