[Proj] does proj.4 work with Hotine Oblique Mercator?

[Mikael Rittri]

Hello Rick.
You need to add

    +no_uoff

to your Proj.4 definition. This is just a flag, so it doesn't need any numerical value.
But it must be included so that you get what EPSG calls "Hotine Oblique Mercator Variant A".
Without it, you get the Variant B instead. One could say that the Hotine Oblique Mercator class has two
distinct construction methods (for historical reasons), and the GIS community hasn't been able
to agree to use only one.

The names Variant A and B are fairly recent EPSG names. In WKT etc., other names for the
variants appear (usually different for different WKT dialects).

See also <http://trac.osgeo.org/proj/ticket/104> http://trac.osgeo.org/proj/ticket/104

(Possibly, with Proj 4.8.0 you should also add

    +gamma=337.25556

for the rectified grid angle, but I think an omitted gamma defaults to the given alpha value,
so an explicit gamma is probably unnecessary in your case.)

Best regards,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com

22 jul 2012 kl. 18:42 skrev "Rick Graves" <gravesricharde at yahoo.com<mailto:gravesricharde at yahoo.com>>:

Hey,

I am not sure that proj.4 is converting latitude and longitude into Hotine Oblique Mercator coordinates correctly.  (But I am a newbie at cartography and such, so please bear with me, I could be making a mistake.)

As a result of the 2010 census in the USA, many states have redrawn their congressional districts.  Michigan is one, and the shapefiles for their new districts were created using the Hotine Oblique Mercator projection to get the whole state onto one map with a minimum of overall distortion.  There is a map and explanation here:

http://www.michigan.gov/documents/DNR_Map_Proj_and_MI_Georef_Info_20889_7.pdf

One can download Michigan's new Congressional district shapefiles from here:

http://www.mcgi.state.mi.us/mgdl/?rel=thext&action=thmname&cid=9&cat=Legislative+Districts+2010+US+Congress

The included prj file has this info:

PROJCS["NAD83 / Michigan Oblique Mercator",
GEOGCS["NAD83",
    DATUM["North_American_Datum_1983",
SPHEROID["GRS 1980",6378137,298.257222101,
   AUTHORITY["EPSG","7019"]],
AUTHORITY["EPSG","6269"]],
    PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.01745329251994328,
AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4269"]],
UNIT["metre",1,
    AUTHORITY["EPSG","9001"]],
PROJECTION["Hotine_Oblique_Mercator"],
PARAMETER["latitude_of_center",45.30916666666666],
PARAMETER["longitude_of_center",-86],
PARAMETER["azimuth",337.25556],
PARAMETER["rectified_grid_angle",337.25556],
PARAMETER["scale_factor",0.9996],
PARAMETER["false_easting",2546731.496],
PARAMETER["false_northing",-4354009.816],
AUTHORITY["EPSG","3078"],
AXIS["X",EAST],
AXIS["Y",NORTH]]

This page has info on the Hotine Oblique Mercator:

http://www.remotesensing.org/geotiff/proj_list/hotine_oblique_mercator.html

Using that, I came up with this command to invoke proj:

proj +proj=omerc +lat_0=45.30916666666666 +lonc=-86 +alpha=337.25556 +k_0=0.9996 +x_0=2546731.496 +y_0=-4354009.816

I get a latitude and longitude for Ann Arbor Michigan of 42 15 N - 83 45 W here:

http://wiki.answers.com/Q/What_is_the_latitude_and_longitude_of_ann_arbor_mi

Plugging 42d15N 83d45W into proj, I get -7945305.71 17799792.28.  But the bounding box of the entire state's shape file is this:

161318.17712499946, 128066.04250000045, 791832.2827499993, 859206.8181249984

The values I get from proj are both out of bounds.

Am I doing something wrong?

Thanks,

Rick Graves

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