Hi,
Answers below
On 2018-11-08 12:10 PM, Morgan Fletcher wrote:
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?
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.
* 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)
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.
* 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?
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.
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
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