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I am working with some older (ca. 1900) USGS Topographic Maps. These Raster maps are georeferenced with a CRS of NAD27 and poly projection (polyconic). However, they are offset and require easting/northing corrections in the georeferenced source data to overlay on modern maps.

On some maps, in the 1920's, Someone had calculated these offsets for the original editions, perhaps to celebrate the creation of NAD27, who knows.

I am looking for some pointers on how that was done.

I want to know if there is a way to calculate this correction generally/automatically. , or alternatively if there is a standard set of datums to use to account for the easting/northing correction in these old maps.

Example

San Francisco 1916 15' x 15', Warning >10MB

This source data is already referenced to NAD27, the CRS as encoded in the file is usually interpreted by QGIS as something like 

+proj=poly +lat_0=37.75 +lon_0=-122.375 +x_0=0 +y_0=0 +datum=NAD27 +units=m +no_defs

While the projection is good, it is not perfectly correct, when used as-is the data is quite offset

enter image description here

Luckily, on some editions the following can be found

enter image description here

Which I Interpret as Easting and Northing corrections to the CRS definition. 

+proj=poly +lat_0=37.75 +lon_0=-122.375 +x_0=-97.536 +y_0=-201.168 +datum=NAD27 +units=m +no_defs

The result is quite good.

enter image description here

Additionally, I have also encountered the following note

enter image description here

By extracting the measured projected distance (in QGIS) between the solid frame and the dashed frame

enter image description here

I can extract x_0 and y_0 and use it like above

Sometimes, however nothing is provided and I calculate this offset by hand by doing some projected distance measurements of good reference points.

In all cases its a labor intensive process.

Last Word

Someone has calculated these offsets before, so I am looking for some pointers on how that was done.

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    It appears to be inaccessible now (Monday, Nov 13 2017) but NADCON 5.0 can convert from NAD27 to older datums. That's a new capability at version 5.
    – mkennedy
    Commented Nov 13, 2017 at 18:36
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    The link to NADCON 5.0 should work now (later on Monday, Nov 13 2017).
    – mkennedy
    Commented Nov 13, 2017 at 21:09

1 Answer 1

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Prior to NAD27 / North American Datum of 1927, US maps used the US standard datum AKA USSD. Starting at NADCON 5.0, the National Geodetic Survey (NGS) has started supporting transformations to and from USSD. You can access the Coordinate Conversion tool online, here.

That link goes to a beta version of the tool. At some point, the online tool will move out of beta (NADCON 5.0 has already been declared out-of-beta). If the link stops working or doesn't redirect properly, try starting at the main page, http://www.ngs.noaa.gov, and hover over Tools in the header and select "Datum Transformations (NADCON)" in the list.

The homepage for NADCON 5.0 includes a link to a PDF that gives more details on the supported datums.

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  • Thank you for the follow up. NADCON provides an ftp repository, with what appears to be the raw tools used to provide this service, have you ever tried or had success using these tools locally instead of as a web service?
    – crasic
    Commented Nov 13, 2017 at 21:30
  • That's great! Good going!
    – mkennedy
    Commented Nov 21, 2017 at 3:55

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