I am new to GIS, an amateur who is curious about old roads. I have QGIS 3.4.0 installed on OS-X 10.13.6 using the pre-built binaries available from https://download.qgis.org/. If I visit the USGS topoView <https://ngmdb.usgs.gov/topoview/> page and download a historic map, for instance the geotiff archive from Tamalpais, CA 1897 (1950 ed.) <https://ngmdb.usgs.gov/topoview/viewer/#10/37.8564/-122.6336>, I can add a raster layer with the .tif file from the archive and it will be placed in rough correspondence with a base map in my QGIS project. In my case my basemap is OpenStreetMap. (crs=EPSG:3857&format&type=xyz&url= http://a.tile.openstreetmap.org/%7Bz%7D/%7Bx%7D/%7By%7D.png&zmax=19&zmin=0) The problem is that the maps don't line up, visually. For example:
Screen Shot 2018-11-02 at 8.51.02 AM.png <https://drive.google.com/a/hahaha.org/file/d/0B-OybZpGWIkHUWhuWEZLS2xWZEhTUC1MMkN4X2dfZGkxQm1J/view?usp=drive_web> I can solve it better with georeferencer. Before I viewed Hans van der Kwast's excellent Georeferencing a scanned map and digitizing vectors in QGIS <https://youtu.be/4IWyVeGhzog> video, I simply started finding common points; now I understand that using the correct CRS, projection and the grid is perhaps perhaps the best strategy. My questions are: - If I download a geotiff archive from topoView, is QGIS 3.4.0 correctly parsing the data in the .tif file, or the other files (.prj, .tfw) in the extracted directory, and placing the map correctly? Can it be adjusted to align better, and if so, how? - Should I use EPSG:26710 for the Tamalpais <https://ngmdb.usgs.gov/topoview/viewer/#10/37.8754/-122.6260> map, and attempt to georeference it myself, using its minute grid? Why does the map border show a skew at the top left and right corners? (see below) - I have found that, to get a hand-drawn, antique map to align with a modern base map, I often have to add so many points in georeferencer that the rendered map has to be very distorted. What is the best method to get old maps to line up, so that historical roads can be related to modern roads? Skew mentioned in second point: Screen Shot 2018-11-08 at 8.20.56 AM.png <https://drive.google.com/a/hahaha.org/file/d/0B-OybZpGWIkHeEE0aDMyQVFCZ0F1R01pRGhKeEJVTzYxaEpj/view?usp=drive_web> I couldn't find a searchable archive of qgis-user; my apologies if these are already answered somewhere. I did ask a variation of these questions on StackExchange <https://gis.stackexchange.com/questions/301199/fix-offset-of-geotiff-file>, where it was heavily edited by 'Vince', and has no answers after a week, so asking here. Morgan
_______________________________________________ Qgis-user mailing list [email protected] List info: https://lists.osgeo.org/mailman/listinfo/qgis-user Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
