It may sound like I know what I am talking about,
but you may not want to make that assumption.
Everything is working fine for you. Older USGS
topo maps (~1980 and earlier) are in NAD27 datum.
Newer maps (~1990 and newer) might be in WGS84,
but I'm not sure about that.
I'm thinking if you change your datum to NAD27
that deviation may go away.
I believe the older files can be changed to WGS84
with GDAL if needed, but I generally just leave them
in NAD27. In MO, where I normally work, the difference
between NAD27 and WGS84 is pretty small - roughly the
amount shown on your maps. I would expect it to be
higher where you are working - near Oakland, CA:
http://www.mflan.com/temp/shift.png
Mike
On 4/19/20 4:41 AM, qgis-user-requ...@lists.osgeo.org wrote:
Date: Sat, 18 Apr 2020 14:43:03 -0700
From: Morgan Fletcher<mor...@hahaha.org>
To:qgis-user@lists.osgeo.org
Subject: [Qgis-user] Dumb USGS geotiff questions
Message-ID:
<capwdt1xd78gqovago1d86ktek0vt0ow2dt_zj+xrecn4anp...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
(Resending, with screenshots as google drive links, to cut down message
size.)
I am running 3.12.1-București on a mac, os-x 10.14.6. My knowledge of GIS
is rudimentary, I've learned what I need to, to satisfy my curiosity about
my region, using old maps.
I have downloaded all the USGS maps for my region - Oakland, CA, USA, which
corresponds to "CA Concord" - from USGS topoView, in geotiff format.
Opening them in QGIS gives fairly wild offsets; they are not all encoded
the same way, yet have common corners. Features on the map vary with what
modern maps show, but getting their corners right would help me a lot.
If I create a new project, set my CRS to WGS 84 / Pseudo-Mercator
(EPSG:3857) as I think is correct for my base map, Google Maps (EPSG:3857 -
WGS 84 / Pseudo-Mercator - Projected) then Layer > Add Layer > Add Raster
Layer > Raster Dataset > CA_Concord_465520_1943_62500_geo.tif > Add, I get
prompted with:
https://drive.google.com/file/d/1FKVPrHLtXXKK8a-QTTz3IigT9dpj1Hou/view?usp=sharing
I don't know what to choose, here. I chose the first one. I've tried
others. The tif.prj file does say
DATUM["D_North_American_1927",SPHEROID["Clarke_1866",6378206.4,294.9786982]],
which I am guessing conforms to NAD27. If I add an opacity slider to the
new raster layer, set its opacity, and then use the Zoom to Coordinate
panel to navigate to 37º45'N 122º15'W, I find that the point is off from
what's on the map, see:
https://drive.google.com/file/d/10vYtcM1FZ-vdS3z1FPW2EYbrNy2sj65L/view?usp=sharing
This is similarly true at the top left:
https://drive.google.com/file/d/18naBKuwk6rZ9DP0tFOZxnGKlIpRQgDKM/view?usp=sharing
Also, note how the map shows two lines at top left, it's true at top right.
This is something to do with projection, I think. I've tried to
georeference these lines, to get them to line up right, but which
intersection do I use, when there are two vertical lines intersecting with
the top, horizontal border, to reference 122º 15' 38º 00' ? I've had mixed
results, trying to use deg-min-ss coordinates to georeference these; I get
a map off the west coast of Africa. Using points on the map is more
successful.
All the maps have slightly variant values in their tfw files. Their prj
files are more uniform, but also show variations. I'd like to normalize
them all, so that I can view the USGS topo maps for my region over time.
Advice?
Thanks,
Morgan
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