Re: [MI-L] Ghana Projection (EPSG 2136)

[David Sherrod]

The Ghana Grid solution may not be trifling, depending on the vintage you are interested in.  It would have been interesting to have seen your tentative version.  The projection info is sometimes given as metric (units = meters, 7) but yours was the more common, using  Gold Coast foot.  GCF is very slightly smaller than international foot. I don't see GCF as a unit of choice in the MapInfo list of options, so I used international foot. The difference is about 3 ppb (per billion). At least one webpage used 4.4 for lat of origin; most use 4.6666666667. I saw one listing with 300,000 as false easting.  I don't know how many local variants there are, but local African projections are always problematic.  Hopefully you have GIS layers that allow some comparison.   Here’s what I conclude.  Transverse Mercator (Gauss Kruger). Your projection line would have these elements, in order Projection: 8 Datum: 136 (Leigon, Clarke 1880) Units: 3 (feet; Gold Coast foot) Origin long: -1.0  (negative) Origin lat: 4.666666666666667  (positive) Scale: 0.99975 False east: 900,000 False north: 0   Something like this? “---Universal Transverse Mercator Ghana grid Clarke 1880---“ “UTM zone 30, Southern Hemisphere (Ghana)”, 8, 136, 3, -1, 4.666666666666667, 0.99975, 900000, 0 --- On Thu, 3/21/13, PSammiCC <sam.c.c...@gmail.com> wrote: From: PSammiCC <sam.c.c...@gmail.com> Subject: [MI-L] Ghana Projection (EPSG 2136) To: mapi...@googlegroups.com Date: Thursday, March 21, 2013, 11:14 AM Hello, Does anyone have the text for the Ghana National Grid projection that I can add to my .PRJ? I have found the details and how to section of the user guide but just seeing if anyoine happened to have it already as I'm not 100% confdent creating it from scratch!? Thanks. -- -- You received this message because you are subscribed to the Google Groups "MapInfo-L" group.To post a message to this group, send email to mapi...@googlegroups.com To unsubscribe from this group, go to: http://groups.google.com/group/mapinfo-l/subscribe?hl=en For more options, information and links to MapInfo resources (searching archives, feature requests, to visit our Wiki, visit the Welcome page at http://groups.google.com/group/mapinfo-l?hl=en   --- You received this message because you are subscribed to the Google Groups "MapInfo-L" group. To unsubscribe from this group and stop receiving emails from it, send an email to mapinfo-l+...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.

[PSammiCC]

Thats great, thanks Dave.

 

I'd gotten as far as:

"--- Ghana Coordinate Systems ---"
"Accra_Ghana_National_Grid", 8, 136?, 3?, -1?, 4.666666667, 0.99975, 900000, 0

 

Where:

"Accra_Ghana_National_Grid",
PROJECTION TYPE= 8,
DATUM= 999?? 136??,
UNITS= 3??,
ORIG LONG= -1??,
ORIGIN LATITUDE= 4.666666667,
SCALE FACTOR= 0.99975,
FALSE EASTING= 900000,
FALSE NORTHING= 0

(Using Chapter 15/Appendix B of the user guide, spatialreference.org and eventually georepository.com)

I agree on 4.6666666667 over 4.4. I struggled with the datum (had defaulted to attempting a custom 999 as all I could see was "Accra" which isn't a MapInfo option) and wanted a second opinion on units and origin long.

 

Frustratingly, I can't check this as I currently don't have any layers to check with yet! I'm also a little surprised MapInfo don't have it as a standard one, to be honest.

 

Many thanks!