topic Re: How to find the unknown coordinates reference system for my map data in Coordinate Reference Systems Questions

How to find the unknown coordinates reference system for my map data

Kofi_Lartey — Mon, 21 Mar 2016 18:13:43 GMT

I have several shapesfiles which perfectly overlay but the coordinates reference system is unknown. I need to overly these with other data in a differetn coordinate system. The coordinate extent is quite unique. I tried initially to overlay with several projected and geographic coordinates systems associated with the area out of which two showed very close displacement (about 60m). Next i tried different datum with one of the 'promising' coordinates reference systems (a geographic coordinates system) and also change the units but same results. Attached is a clip of two of the coordinate extents. I am yet to try the second 'promising' CRS but decided to put it out just in case i am missing something

Re: How to find the unknown coordinates reference system for my map data

AdrianWelsh — Mon, 21 Mar 2016 18:44:25 GMT

Hi Kofi,

Have you tried using the Georepository website to find common coordinate systems for your area?

Geodetic Parameter Database

This may help in finding what you need.

Re: How to find the unknown coordinates reference system for my map data

RebeccaStrauch__GISP — Mon, 21 Mar 2016 21:02:41 GMT

I have several shapesfiles which perfectly overlay

What are the overlaying perfectly to? If a basemap or a raster, does that have a coordinate system identified? If so, you could look at the properties of that layer, save the coordinate system as a "favorite" then in ArcCatalog, update the properties using this favorite coordinate system. This is not projecting but just assigning a system....and should only be used only in the situation I mentioned. (note: you could also see if the data frame has a coordinate system assigned, this may project data with coordinate systems assigned, but will not project those items with unknown.)

This may be more confusing that helpful.

Otherwise, I would refer you to what Adrian suggested.

Re: How to find the unknown coordinates reference system for my map data

MelitaKennedy — Mon, 21 Mar 2016 21:12:53 GMT

Could this data have come from a converted raster? The -0.5 value is classic for a raster that hasn't been georeferenced. What coordinate systems are within 60 meters?

Re: How to find the unknown coordinates reference system for my map data

Kofi_Lartey — Tue, 22 Mar 2016 01:37:35 GMT

Rebecca, all of them overlay but have unknown coordinates, (eg shpfile_1 top 3576.5; left -0.5; bottom -0.5; right 2455.5: shpfile_2 top 3566.5; left -0.5; bottom 3.5; right 2455.5). for the second 'favorite' too i have tried several geographic coordinates, and changed units to Km, one of them yielded a major shift but not enough to overlay.

Re: How to find the unknown coordinates reference system for my map data

Kofi_Lartey — Tue, 22 Mar 2016 01:43:05 GMT

Ghana Meter Grid and WGS_1984, these two the first projected and the other geographic lies within 50m. I cannot tell how they were developed, but some are rainfall iso, soil maps, vegetation maps etc

Re: How to find the unknown coordinates reference system for my map data

Kofi_Lartey — Tue, 22 Mar 2016 03:52:58 GMT

Ya i have, maybe there is something i am missing, cannt just figure it out.

Re: How to find the unknown coordinates reference system for my map data

NeilAyres — Tue, 22 Mar 2016 06:46:39 GMT

Attached is the doc from ASPRS / Clifford Mugnier details Ghana's geodetic control and history.

I see that your data appears to be old coverage export files. Perhaps you should import them into a fgdb first, before trying to get the underlying GCS / datum correct.

The doc (& the ArcGIS documentation) says that the classical datum in Ghana is Leigon (WKID 1159) and there is a geocentric transformation available in the system.

If you have the data already matching to within 60m, then you obviously have the correct projection system already. Just need to apply this transformation and see if the alignment improves.

Also in the doc, CM details a 7 param Bursa Wolfe transformation which he says greatly improves the accuracy. This is not in the ArcGIS system (at 10.3.1), so you would have to try this by creating a custom transformation.

Re: How to find the unknown coordinates reference system for my map data

Kofi_Lartey — Tue, 22 Mar 2016 18:15:20 GMT

Neil, I must say although i have 'self-thought' to do a lot with my data, i am a bit of a novice. But you are right, the data is old as it was in the .E00 format which i converted to shape file formate using the conversion tool in toolbox in arc catalog before opening in arcmap. I will also follow your suggestion and see what i get. Anyway this information is quite interesting. I will have to figure out how to make it useful.

Re: How to find the unknown coordinates reference system for my map data

NeilAyres — Tue, 22 Mar 2016 19:40:59 GMT

Love the story about the early surveyors tying ropes to pigs tails....

Re: How to find the unknown coordinates reference system for my map data

Kofi_Lartey — Wed, 23 Mar 2016 02:40:30 GMT

Neil, i followed up on it using two transformation systems GCS_north_America_1927 and the 1973 version. Using the +- X as indicated by Capt Wood, but the same results. The only change was when i changed the units from m to Km, but again this is not enough to overlay. My final try is to contact the source of the data and hope i get somebody to respond. Thanks for support

Re: How to find the unknown coordinates reference system for my map data

Kofi_Lartey — Wed, 23 Mar 2016 02:43:35 GMT

Melita, if it was developed from a raster that was not georeferenced, then how come all of them overlay?

Re: How to find the unknown coordinates reference system for my map data

DanPatterson_Retired — Wed, 23 Mar 2016 03:12:01 GMT

If the data are for Ghana, you would be advised to use transformations appropriate for that area rather than those associated with North America

Re: How to find the unknown coordinates reference system for my map data

NeilAyres — Wed, 23 Mar 2016 06:25:49 GMT

Kofi,

I think you had better start again. NAD certainly isn't appropriate for your data (Ghana).

What is the coordinate system of your modern reference data?

Navigate to a feature class using ArcCatalog, right click on it to get the properties page, look at the coord tab.

What does it say there. Please post a snap shot.

The old e00 data, import it into a file geo db please. Do the same, what are the extents, what do the numbers look like (ie small numbers, maybe degrees, or larger numbers, probably some sort of projected metres.

And please don't mess with units (km), until we have the basics sorted out.

Re: How to find the unknown coordinates reference system for my map data

RebeccaStrauch__GISP — Wed, 23 Mar 2016 15:27:11 GMT

Kofi,

It may be time to try to look at Fundamentals of georeferencing a raster dataset—Help | ArcGIS for Desktop

and find some well known locations on an image or data set that does have the projection you want, and the data that is unknown. You can create links between the sets and have the new data rubbersheet to the known layer.

It may be that the coordinate system that was used with the .e00 datasets was a totally "local" system. You asked Melita above,why they all would line up line up if there wasn't a coordinate/reference system.....one possibility (and very likely since the data was in an .e00 format which is the ArcInfo Coverager exchange format) is one of the layers, maybe the imagery, was scanned or digitized in local coordinates, and everything was set to match. That would explain why they match up.

One other thing to check, since it is an old coverage, once you import the coverage....check the folder on the disc for a .prj or similar file. The projection info may reside in the file but since it is the older format, ArcMap/Catalog will not recognize it. However, it may give you a clue as to the parameters used....which may help in tracking down the coordinate system or project, IF on was used (and not local as listed above).

Re: How to find the unknown coordinates reference system for my map data

Kofi_Lartey — Wed, 23 Mar 2016 17:22:17 GMT

Dan, you may be right. ArcGIS gave three alternatives in the convert from (Geographic coordinates system transformation) under Data frame properties, the first was Leigon which is the geographic RS for Ghana Meter Grid, accordingly these will be the set from which a transformation could be carried out. So thats how come i chose to work with the later two which are north A coordinates. I did not however try it at all with the Leigon, i will and see what happens.

Re: How to find the unknown coordinates reference system for my map data

Kofi_Lartey — Wed, 23 Mar 2016 17:24:32 GMT

Neil, thanks for that eye opener, you are very correct, i should have tried transformations between Leigon and others associated with Ghana rather than the North American.

Re: How to find the unknown coordinates reference system for my map data

MelitaKennedy — Thu, 24 Mar 2016 20:25:52 GMT

They may overlay each other because they cover the same area, but are in "raw" meter-based values. The numbers don't make sense for either Ghana grid (Accra or Leigon GCS/datums) that's in ArcGIS. Both should have all positive easting values. If they are in meters, should the data extent be 2.5 km by 3.5 km? What location / approximate area should these datasets represent?

Melita

Re: How to find the unknown coordinates reference system for my map data

Kofi_Lartey — Thu, 24 Mar 2016 23:59:03 GMT

Rebecca, l followed your advice and that from others and have made progress. Yes i used the conversion tool in the Toolbox in Arc catalog and from the output i obtained the coordinate reference system information: I built a custom projection system and all the layers overlaid. Know i am using the project tool in Arcmap to change the coordinates to unify it with the rest of my data, and that is taking two days, and still counting. I think i will post details of the processes i used when i am done.

Re: How to find the unknown coordinates reference system for my map data

Kofi_Lartey — Fri, 25 Mar 2016 00:07:28 GMT

All the comments have been useful , i think i have made progress. When i am done i will post the stages involved. I have been able to identify the unknown coordinates system, I built a custom projection system and assigned a coordinates projection system to the data. I am now using the project tool to change the coordinates to unify with the rest of my data. That is taking a bit too long though (two days and still counting).