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<p>Hi,</p>
<p>Answers below<br>
</p>
<br>
<div class="moz-cite-prefix">On 2018-11-08 12:10 PM, Morgan Fletcher
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAPwdt1U+TmE7cJR4GF63E13MPaPYWO9nsbzocHj2R8nG80duhw@mail.gmail.com">
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<div dir="ltr">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 <a
href="https://download.qgis.org/" target="_blank"
moz-do-not-send="true">https://download.qgis.org/</a>. If I
visit the <a href="https://ngmdb.usgs.gov/topoview/"
target="_blank" moz-do-not-send="true">USGS topoView</a> page
and download a historic map, for instance the geotiff archive
from <a
href="https://ngmdb.usgs.gov/topoview/viewer/#10/37.8564/-122.6336"
target="_blank" moz-do-not-send="true">Tamalpais, CA 1897
(1950 ed.)</a>, 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=<a
href="http://a.tile.openstreetmap.org/%7Bz%7D/%7Bx%7D/%7By%7D.png&zmax=19&zmin=0"
moz-do-not-send="true">http://a.tile.openstreetmap.org/%7Bz%7D/%7Bx%7D/%7By%7D.png&zmax=19&zmin=0</a>)
The problem is that the maps don't line up, visually. For
example:</div>
<div dir="ltr"><br>
</div>
<div dir="ltr">
<div><br>
<div class="gmail_chip gmail_drive_chip"
style="width:396px;height:18px;max-height:18px;background-color:#f5f5f5;padding:5px;color:#222;font-family:arial;font-style:normal;font-weight:bold;font-size:13px;border:1px
solid #ddd;line-height:1"><a
href="https://drive.google.com/a/hahaha.org/file/d/0B-OybZpGWIkHUWhuWEZLS2xWZEhTUC1MMkN4X2dfZGkxQm1J/view?usp=drive_web"
target="_blank"
style="display:inline-block;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;text-decoration:none;padding:1px
0px;border:none;width:100%" moz-do-not-send="true"><img
style="vertical-align: bottom; border: none;"
src="https://drive-thirdparty.googleusercontent.com/16/type/image/png"
moz-do-not-send="true"> <span dir="ltr"
style="color:#15c;text-decoration:none;vertical-align:bottom">Screen
Shot 2018-11-02 at 8.51.02 AM.png</span></a></div>
<br>
</div>
<div><br>
</div>
<div>I can solve it better with georeferencer. Before I viewed
Hans van der Kwast's excellent <a
href="https://youtu.be/4IWyVeGhzog" target="_blank"
moz-do-not-send="true">Georeferencing a scanned map and
digitizing vectors in QGIS</a> 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. </div>
<div><br>
</div>
<div>My questions are:</div>
<div><br>
</div>
<div>
<ul>
<li style="margin-left:15px">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?<br>
</li>
</ul>
</div>
</div>
</div>
</blockquote>
The only way to do that is to use the Georeferencer and to start the
job over. Manually changing the tfw files (simple text file you can
open with notepad) would require that you know what the errors are.<br>
<br>
<blockquote type="cite"
cite="mid:CAPwdt1U+TmE7cJR4GF63E13MPaPYWO9nsbzocHj2R8nG80duhw@mail.gmail.com">
<div dir="ltr">
<div dir="ltr">
<div>
<ul>
<li style="margin-left:15px">Should I use EPSG:26710 for
the <a
href="https://ngmdb.usgs.gov/topoview/viewer/#10/37.8754/-122.6260"
target="_blank" moz-do-not-send="true">Tamalpais</a> 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) <br>
</li>
</ul>
</div>
</div>
</div>
</blockquote>
You should use the CRS that is closer to what your historical map is
made from. Generally speaking, if the map was made from ground
surveyors using chains or sticks, I find that a local UTM is
better. Maps in Long lat (like WGS84) are sometimes better for
measurements made from stars. One way to find out is to start with
a simple Helmert transformation with two points. It will help you
see the differences and find the best projection. Then, fine tune
with Thin Plate Spline. The skew is due to the fact that local UTM
coordinates (projected coordinate systems) are in meters or feet
and that the grid is square when looked at in that projection. In
Geographic Coordinate systems, grid is close to a rectangle. All
CRS have distortions. <br>
<blockquote type="cite"
cite="mid:CAPwdt1U+TmE7cJR4GF63E13MPaPYWO9nsbzocHj2R8nG80duhw@mail.gmail.com">
<div dir="ltr">
<div dir="ltr">
<div>
<ul>
<li style="margin-left:15px">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?</li>
</ul>
</div>
</div>
</div>
</blockquote>
Find the best CRS then fine tune with Thin Plate spline. Start by
working far way from the area of interest then work yourself in. <br>
<br>
<blockquote type="cite"
cite="mid:CAPwdt1U+TmE7cJR4GF63E13MPaPYWO9nsbzocHj2R8nG80duhw@mail.gmail.com">
<div dir="ltr">
<div dir="ltr">
<div>
<div>Skew mentioned in second point:</div>
</div>
<div><br>
</div>
<div>
<div><br>
<div class="gmail_chip gmail_drive_chip"
style="width:396px;height:18px;max-height:18px;background-color:#f5f5f5;padding:5px;color:#222;font-family:arial;font-style:normal;font-weight:bold;font-size:13px;border:1px
solid #ddd;line-height:1"><a
href="https://drive.google.com/a/hahaha.org/file/d/0B-OybZpGWIkHeEE0aDMyQVFCZ0F1R01pRGhKeEJVTzYxaEpj/view?usp=drive_web"
target="_blank"
style="display:inline-block;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;text-decoration:none;padding:1px
0px;border:none;width:100%" moz-do-not-send="true"><img
style="vertical-align: bottom; border: none;"
src="https://drive-thirdparty.googleusercontent.com/16/type/image/png"
moz-do-not-send="true"> <span dir="ltr"
style="color:#15c;text-decoration:none;vertical-align:bottom">Screen
Shot 2018-11-08 at 8.20.56 AM.png</span></a></div>
<br>
</div>
</div>
<div><br>
</div>
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 <a
href="https://gis.stackexchange.com/questions/301199/fix-offset-of-geotiff-file"
moz-do-not-send="true">StackExchange</a>, where it was
heavily edited by 'Vince', and has no answers after a week, so
asking here.<br>
<br>
</div>
<div dir="ltr">Morgan</div>
</div>
<br>
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