[Qgis-user] Converting X Y Coordinates to Latitude/Longitude

[Tudorache, Marian]

Nicholas method is about transforming coordinates from a CRS to another one.

QGIS also has the possibility of enable on the fly CRS transformation.

In both cases you should know the CRS for vectorized historic map.

QGIS will do the best to generate a custom CRS and associate to your data but it does not work all the time.

Do you know the CRS for vectorized historic map?

To be accurate during coordinate transformation you need to associated the original data with a CRS.

We had an issue with not georeferenced data. When we loaded the vectorized data, QGIS assigned a weird CRS. After we <<save as>> to GRS80 the area was shifted.
We knew the area was somewhere in Vancouver and the area was over a stereographic projection. I knew the point of tangency and I created a custom CRS which was associated to my original data.

After this tweak we were able to overlay precisely the area where it belongs.

I hope this helps.
Marian


From: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of Nicolas Cadieux
Sent: June-07-18 9:54 PM
To: David Hochfelder
Cc: qgis-user at lists.osgeo.org
Subject: Re: [Qgis-user] Converting X Y Coordinates to Latitude/Longitude

Hi David,

I’am an archaeologist working in Québec. If you vectorized the map in a local CRS, all you need to do is do a « save as » and change the CRS (coordinate reference system) that is in Longitude Latitude.  If your map is not georefenrenced (that is, you just scanned the map and did not use a CRS, that is more complicated but not impossible.  I usually fixe that do in a CAD but I am sure you can do it in QGIS.  We could talk if you want and I could help you out figured the next steps and steps to avoid this problem (like georefereing the raster map before you start) .Send me an email directly.

You could look at this also.
https://gis.stackexchange.com/questions/136817/qgis-moving-the-layer-all-features-using-vector-affine-transformation

Nicolas

Le 7 juin 2018 à 20:49, David Hochfelder <dhochfelder at albany.edu<mailto:dhochfelder at albany.edu>> a écrit :
Hi All,

I am new to QGIS, so this may be a newbie question easily answered by the more experienced.

I'm working on a historical GIS project to remap an area of Albany, NY demolished for urban renewal in 1962. I've succeeded in vectorizing a historic map of the area (~1960), but the coordinates are not in lat/long form. I see that there are plugins for converting tables of specific coordinates, but I want to convert the entire map continuously.

This area no longer exists--all the buildings and streets were cleared for the Empire State Plaza state capitol complex.

My end goal is to use the rectified map as a way to geotag historic photographs of particular buildings.

Is there a way to convert the coordinates in a continuous fashion for the entire region?

Thanks for your help.

Dave Hochfelder


David Hochfelder
Associate Professor
History Department
Social Science 060B
University at Albany, SUNY
Albany, NY 12222
518-442-5348
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