# [Proj] Ortho Projection Question

Brian Krzys cokrzys at gmail.com
Thu Nov 3 20:37:03 EST 2011

```I am trying to duplicate an ESRI "local" projection with Proj.4
parameters and can't quite get it worked out.  The ESRI custom
projection file has:

PROJCS["Twin_Creeks_Mine_Grid (Local)",
GEOGCS["GCS_North_American_1927",
DATUM["D_North_American_1927",SPHEROID["Clarke_1866",6378206.4,294.9786982]],
PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
PROJECTION["Local"],
PARAMETER["False_Easting",14225.0],
PARAMETER["False_Northing",36603.0],
PARAMETER["Scale_Factor",1.0],
PARAMETER["Azimuth",0.0],
PARAMETER["Longitude_Of_Center",-117.183175],
PARAMETER["Latitude_Of_Center",41.217358],
UNIT["Foot_US",0.3048006096012192]]

The equivalent Proj.4 parameters I've setup are:

+proj=ortho +datum=NAD27 +lat_0=41.217358 +lon_0=-117.183175
+x_0=14225.0 +y_0=36603.0 +units=us-ft no_defs

I want to convert to lat-long WGS84 so I am going to this:

+proj=longlat +datum=WGS84 +no_defs no_defs

My input test point is:

x = 15768.1900000000 y = 65384.1600000000

In ESRI land I get back (lat-long): 41.160175 -117.177571

With Proj.4 I am getting: 41.0674232619472 -117.296349865729

I've also tried this with a couple of specific +to_wgs transformations
on the ESRI and Proj.4 side and still cannot get the conversions to
match.

This is all with Proj.4 Rel. 4.7.1, 23 September 2009.

What I'm wondering about is the ortho "projection" and if there is
maybe a better option to use?  I've searched around the web a bit and
read about concerns with ortho so also not sure if there is a proj
update beyond 4.7.1 that might help?  I know this whole kind of
direct-Cartesian conversion isn't the best thing, but generally stuck
just trying to match what Arc is doing.

Thanks for the help, Brian.
```