<div dir="auto"><div data-smartmail="gmail_signature" dir="auto">Looking at <br></div><div class="gmail_quote" dir="auto"><div dir="auto"><div dir="auto"><br></div><div dir="auto"><a href="https://ngmdb.usgs.gov/img4/ht_icons/Browse/CA/CA_Tamalpais_301790_1897_62500.jpg" rel="noreferrer noreferrer noreferrer" target="_blank">https://ngmdb.usgs.gov/img4/ht_icons/Browse/CA/CA_Tamalpais_301790_1897_62500.jpg</a></div><div dir="auto"><br></div><div dir="auto">there is a note in the lower right corner:</div><div dir="auto"><br></div><div dir="auto">Polyconic projection. To place on North American datum, move projection lines 690 ft south and 320 ft west.</div><div dir="auto"><br></div><div dir="auto"><a href="https://en.m.wikipedia.org/wiki/American_polyconic_projection" rel="noreferrer noreferrer noreferrer" target="_blank">https://en.m.wikipedia.org/wiki/American_polyconic_projection</a></div><div dir="auto"><br></div><div dir="auto">explains the projection.</div><div dir="auto"><br></div><div dir="auto">Unfortunately, the map does not mention which central meridian was chosen for the projection. It may be possible to find if at the time a single one was used for all of CA and what it would be.</div><div dir="auto"><br></div><div dir="auto">It should also be straightforward to determine if proj and therefore qgis supports the polyconic projection. It probably does.</div><div dir="auto"><br></div><div dir="auto">With that you can define a custom projection. Perhaps there is an EPSG code but I could not find it.</div><div dir="auto"><br></div><div dir="auto">EPSG 9818 seems to encode this projection although it does not seem to include the actual method in the registration.</div><div dir="auto"><br></div><div dir="auto">After you have defined a closely matching projection, you can translate the minute grid points to projected coordinates and georeference.</div><div dir="auto"><br></div><div dir="auto">The other mystery is the note about the datum shift. What are projection lines and what datum is used for the printed map ? The mentioned North American datum may be NAD27.</div><div dir="auto"><br></div><div dir="auto">Andreas</div><div dir="auto"><br></div><div dir="auto"><br><div data-smartmail="gmail_signature" dir="auto">From: Morgan Fletcher <<a href="mailto:morgan@hahaha.org" rel="noreferrer noreferrer noreferrer noreferrer" target="_blank">morgan@hahaha.org</a>></div><div class="gmail_quote" dir="auto"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
To: <a href="mailto:qgis-user@lists.osgeo.org" rel="noreferrer noreferrer noreferrer noreferrer" target="_blank">qgis-user@lists.osgeo.org</a><br>
Subject: [Qgis-user] Aligning georeferenced USGS historical maps<br>
Message-ID:<br>
<<a href="mailto:CAPwdt1U%2BTmE7cJR4GF63E13MPaPYWO9nsbzocHj2R8nG80duhw@mail.gmail.com" rel="noreferrer noreferrer noreferrer noreferrer" target="_blank">CAPwdt1U+TmE7cJR4GF63E13MPaPYWO9nsbzocHj2R8nG80duhw@mail.gmail.com</a>><br>
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<br>
I am new to GIS, an amateur who is curious about old roads. I have QGIS<br>
3.4.0 installed on OS-X 10.13.6 using the pre-built binaries available from<br>
<a href="https://download.qgis.org/" rel="noreferrer noreferrer noreferrer noreferrer noreferrer" target="_blank">https://download.qgis.org/</a>. If I visit the USGS topoView<br>
<<a href="https://ngmdb.usgs.gov/topoview/" rel="noreferrer noreferrer noreferrer noreferrer noreferrer" target="_blank">https://ngmdb.usgs.gov/topoview/</a>> page and download a historic map, for<br>
instance the geotiff archive from Tamalpais, CA 1897 (1950 ed.)<br>
<<a href="https://ngmdb.usgs.gov/topoview/viewer/#10/37.8564/-122.6336" rel="noreferrer noreferrer noreferrer noreferrer noreferrer" target="_blank">https://ngmdb.usgs.gov/topoview/viewer/#10/37.8564/-122.6336</a>>, I can add a<br>
raster layer with the .tif file from the archive and it will be placed in<br>
rough correspondence with a base map in my QGIS project. In my case my<br>
basemap is OpenStreetMap. (crs=EPSG:3857&format&type=xyz&url=<br>
<a href="http://a.tile.openstreetmap.org/%7Bz%7D/%7Bx%7D/%7By%7D.png&zmax=19&zmin=0" rel="noreferrer noreferrer noreferrer noreferrer noreferrer" target="_blank">http://a.tile.openstreetmap.org/%7Bz%7D/%7Bx%7D/%7By%7D.png&zmax=19&zmin=0</a>)<br>
The problem is that the maps don't line up, visually. For example:<br>
<br>
<br>
Screen Shot 2018-11-02 at 8.51.02 AM.png<br>
<<a href="https://drive.google.com/a/hahaha.org/file/d/0B-OybZpGWIkHUWhuWEZLS2xWZEhTUC1MMkN4X2dfZGkxQm1J/view?usp=drive_web" rel="noreferrer noreferrer noreferrer noreferrer noreferrer" target="_blank">https://drive.google.com/a/hahaha.org/file/d/0B-OybZpGWIkHUWhuWEZLS2xWZEhTUC1MMkN4X2dfZGkxQm1J/view?usp=drive_web</a>><br>
<br>
<br>
I can solve it better with georeferencer. Before I viewed Hans van der<br>
Kwast's excellent Georeferencing a scanned map and digitizing vectors in<br>
QGIS <<a href="https://youtu.be/4IWyVeGhzog" rel="noreferrer noreferrer noreferrer noreferrer noreferrer" target="_blank">https://youtu.be/4IWyVeGhzog</a>> video, I simply started finding common<br>
points; now I understand that using the correct CRS, projection and the<br>
grid is perhaps perhaps the best strategy.<br>
<br>
My questions are:<br>
<br>
<br>
- If I download a geotiff archive from topoView, is QGIS 3.4.0 correctly<br>
parsing the data in the .tif file, or the other files (.prj, .tfw) in the<br>
extracted directory, and placing the map correctly? Can it be adjusted to<br>
align better, and if so, how?<br>
- Should I use EPSG:26710 for the Tamalpais<br>
<<a href="https://ngmdb.usgs.gov/topoview/viewer/#10/37.8754/-122.6260" rel="noreferrer noreferrer noreferrer noreferrer noreferrer" target="_blank">https://ngmdb.usgs.gov/topoview/viewer/#10/37.8754/-122.6260</a>> map, and<br>
attempt to georeference it myself, using its minute grid? Why does the map<br>
border show a skew at the top left and right corners? (see below)<br>
- I have found that, to get a hand-drawn, antique map to align with a<br>
modern base map, I often have to add so many points in georeferencer that<br>
the rendered map has to be very distorted. What is the best method to get<br>
old maps to line up, so that historical roads can be related to modern<br>
roads?<br>
<br>
Skew mentioned in second point:<br>
<br>
<br>
Screen Shot 2018-11-08 at 8.20.56 AM.png<br>
<<a href="https://drive.google.com/a/hahaha.org/file/d/0B-OybZpGWIkHeEE0aDMyQVFCZ0F1R01pRGhKeEJVTzYxaEpj/view?usp=drive_web" rel="noreferrer noreferrer noreferrer noreferrer noreferrer" target="_blank">https://drive.google.com/a/hahaha.org/file/d/0B-OybZpGWIkHeEE0aDMyQVFCZ0F1R01pRGhKeEJVTzYxaEpj/view?usp=drive_web</a>><br>
<br>
<br>
I couldn't find a searchable archive of qgis-user; my apologies if these<br>
are already answered somewhere. I did ask a variation of these questions on<br>
StackExchange<br>
<<a href="https://gis.stackexchange.com/questions/301199/fix-offset-of-geotiff-file" rel="noreferrer noreferrer noreferrer noreferrer noreferrer" target="_blank">https://gis.stackexchange.com/questions/301199/fix-offset-of-geotiff-file</a>>,<br>
where it was heavily edited by 'Vince', and has no answers after a week, so<br>
asking here.<br>
<br>
Morgan<br>
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