Is it possible to define a custom CRS for a shapefile in QGIS that uses hundredths of a US foot?

Question

I have several polygon shapefiles whose coordinates are defined as 100x their original coordinates projected from the Washington State South HARN State Plane in us-ft (EPSG 2927). I would imagine the purpose of this scheme is to represent more precise coordinates in integer form. Unfortunately, I am having a hard time representing this custom coordinate system as a proj4 specification in QGIS. Each of these shapefiles has a .prj file and no .qpj file.

Things I have tried or thought of:

1. creating a custom proj file by editing the +units parameter to "us-ft/100" (not defined).
2. the .prj file has a parameter called "scale-factor" that I tried to set to 0.01, but QGIS does not appear to read this (not sure if the .prj file is being ignored or just this parameter).
3. using a geoalgorithm to "rescale" each feature in the shapefile by 0.01, so that they actually fit within the standard EPSG 2927 CRS.
4. Going back to the original application (Geomedia) to reproject the shapefile, which I am finding is difficult with shapefiles. I would also just like to know if there is a more "open source" way of doing this.

What is the best way to accomplish this task in QGIS?

Answer 1

Your first idea is almost working. It needs a scaling factor between custom units and meters:

Copy the definition of EPSG:2927

+proj=lcc +lat_1=47.33333333333334 +lat_2=45.83333333333334 +lat_0=45.33333333333334 +lon_0=-120.5 +x_0=500000.0001016001 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs

to clipboard, and paste it into a new custom CRS. then replace +units with +to_meter:

+proj=lcc +lat_1=47.33333333333334 +lat_2=45.83333333333334 +lat_0=45.33333333333334 +lon_0=-120.5 +x_0=500000.0001015998 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +to_meter=0.00304800609601219 +no_defs

You can find +to_meter values for all accepted units in this ticket:

http://hub.qgis.org/issues/9414

Note that the proj.4 values for +x_0 and +y_0 are always in meters, while the same data in WKT is in the units of the projection. So you have to change these values in a custom WKT too:

PROJCS["Lambert_Conformal_Conic",
  GEOGCS["GCS_GRS 1980(IUGG, 1980)",
  DATUM["D_unknown",
  SPHEROID["GRS80",6378137,298.257222101]],
  PRIMEM["Greenwich",0],
  UNIT["Degree",0.017453292519943295]],
PROJECTION["Lambert_Conformal_Conic"],
  PARAMETER["standard_parallel_1",47.33333333333334],
  PARAMETER["standard_parallel_2",45.83333333333334],
  PARAMETER["latitude_of_origin",45.33333333333334],
  PARAMETER["central_meridian",-120.5],
  PARAMETER["false_easting",164041666.7],
  PARAMETER["false_northing",0],
  UNIT["unknown",0.00304800609601219]]