Commons talk:British Library/Mechanical Curator collection/georeferencing campaign
Hi, I was wondering how (and if) the clipping of the maps works, I was not able to find info on this, but for the maps where I tried to clip I ended up getting requests to clip inside of the actual map (and I did not manage to delete points which were wrong).
- @Basvb: Hi Bas! Thank you so much for trying it out -- sorry you've been having problems with this. Did you not manage to get any maps that you were working on to save then?
- I don't know if clipping is really the right word -- it's more like drawing a bounding box around the area of interest. (Note that there's usually no need to be quite as picky as was done for the polygon in the video). One key thing is to make sure the polygon has closed up -- which you should be able to do by double-clicking the last point (one way to check is by then trying to drag it, and seeing whether everything stays closed up). Then (if I remember correctly) you perhaps have to click the big red 'save' button again to confirm.
- Once you have defined a bounding polygon, and are back in the 'Visualise' tab, selecting or deselecting "cutline" should turn on or off the clipping to the polygon.
- I haven't been having any trouble; but my internet connection tonight is a bit too erratic to try, wavering at one bar which flickers an and off. Let me know if you're having any more luck (and maybe the URLs for some of the maps you've been working on) & I'll try to take a look tomorrow (Wednesday).
- All best, Jheald (talk) 22:46, 14 April 2015 (UTC)
I slowly have been tagging some of the Low Countries' maps. However with this map of the world I run into problems. It gets distorted quite a bit, while it seems to be tagged correctly. Also the accuracy tag does not display the transformation map. Is this a known issue or did I do something weird in this case? Mvg, Basvb (talk) 17:37, 19 April 2015 (UTC)
- While I'm here I have another question. This image has multiple maps displayed. Is there a way to indicate that there are multiple maps in an image. In this case there is a clear main map, in other cases I've seen images of multiple maps with the same importance. Mvg, Basvb (talk) 17:39, 19 April 2015 (UTC)
- @Basvb: Hi Bas. Where there are multiple maps, we've tried to split them into multiple images with one map each. If you go to the "This map" tab, then "view original web presentation" is small print at the bottom left, that will take you to this image on Flickr. In this case following the "Set of maps found within image 11194920364" album link takes you to this Flickr page, showing that the split was done. The intention was that only the split-out maps should have been put on to the Georeferencing system. However, due to a communication mistake, and a rush to finalise things without checking before various people went away, the original unsplit image was also included (which is the one you have just done).
- There are also images which at that point had not yet been split, but now have been. The way it works is that we need to send all the changes and corrections to Klokan in one go. So I hope to remove the images that have been split from the set, and also add new additional split-out images. (And also remove some maps of the world and maps of hemispheres that don't georeference very well, and were not intended to have been included). But it may take a couple more weeks to get that sorted out (and some time for Klokan in Switzerland to put the changes into effect); so in the meantime all I can suggest for the present is: don't, for the time being, do any more images with multiple maps, nor any images of the whole world. I'm really sorry we didn't get it right first time.
- There's also a particular problem on the version we're using (v.3) of Georeferencer with points that cross the International Date Line. The eastern tip of Siberia ought to be getting mapped to about 190 degrees East, continuous with the rest of Siberia. But instead it gets mapped to 170 degrees West. You could actually see this if you changed the visualisation mode from affine transformation to the more flexible TPS ("thin plate spline"). (Warning: this may crash Firefox; but seems okay on Chrome). Then one could actually see the map trying to turn itself inside out to match 170 degrees West for that tip of Siberia. So I have deleted that point, and that seems to have fixed the problem. Jheald (talk) 18:54, 19 April 2015 (UTC)
- Now I remember that I already connected the separated parts of that map. So I'll leave the multiple-part maps be for now. And will leave the world ones if they keep on going like this. No problems about the issues, it's just that it is good to know where the limits are. Mvg, Basvb (talk) 19:29, 19 April 2015 (UTC)
Getting back to Flickr from the Georeferencer
Not exactly prominent, but in small text in the left-hand column of the "This map" tab of the Georeferencer, there's a link to "View: Original web presentation", which can be used to get back from the Georeferencer to the Flickr page.
In turn the Flickr page will usually contain a link to download a pdf of the whole book, which may be the only way to work out what an image is of; and also a tag of the form "other_half:pair=..." which can be used to get to the other half of a two-page spread. Jheald (talk) 23:08, 30 April 2015 (UTC)
Georeferencing against previously-georeferenced maps
... is awkward, but not completely impossible.
See eg these two maps of Roman Manchester for an example.
The trick (I think) is to have open two copies of georeferencer for the image, one is 'georeference' mode and one in 'visualise' mode. If you have previously visualised another map of the same area, you can save that in the favorites section in the sidebar; then later drag it out, and put it in the second slot at the top of the visualiser window (as in the link above). You can then fade up or down either map in the visualiser. This means you can then work out where the point you're trying to match to on the already georeferenced map corresponds to on a modern map, and then use that knowledge in the georeferencing window.