custom projection issue and adding data

[NigelBardoe]

Hi all,

I am not great with projections and I have some data I need to correct.   I am doing work for St. Kitts and Nevis (Caribbean) and the data I got from them has a custom projection.   I asked them for information on it to try to convert it back to something mainstream so I can then use ArcMap to then transform it and was told the custom projection was WGS 1984.

I then converted some lat / long data into decimal degrees and added it to the map.  The resulting points were almost 2 million metres off.  This figure correlates to the massive extent range that the custom projection has (pic attached).  Also, the custom projection shows a GCS of "GCS_North_American_1983", which does not match what they told me. 

I have been attempting to fix this for days now by reading others questions, so any help would be appreciated.

Thanks,

Nigel

[NigelBardoe]

I would just like to add that if there is any additional information that would better enable someone to help me, please let me know and I will try to provide it.

[DanPatterson_Retired]

That looks like a modified transverse Mercator (3 degree utm-ish).  Does the central meridian of -62.5 make sense given your longitude? and if so, you don't define it as a wgs84 which is a geographic coordinate system units in decimal degrees.  So if you have some coordinates in Longi/latitude, then you define it as a GCS WGS84, then use the Project tool to project it... never use the define projection tool to do this task

[JayantaPoddar]

Could you attach a sample data (zipped) from the original dataset for us to review?

[NigelBardoe]

Thanks for the replies Dan and Jayanta.

First, while getting some sample of the point data I realized that they gave me the wrong values in the wrong field.  Latitude degrees in the longitude field.   After correcting the data, the points show up onto my coastline data that was in the St.Kitts and Nevis projection.

Dan:  The -62.5 central meridian does make sense as the island is very close to that longitude.  Now that my data lines up, is it worth changing the shapefile data's projection to something more mainstream?   Would it be as simple as putting the central meridian back to 0?  I will be the only one using the data and only short term, so I'm not sure.

I have attached the coastline layer which has the modified projection.  No sample points as I found that issue.

[DanPatterson_Retired · Accepted solution]

Do not alter anything.  The longitude and latitude data should be defined as a Geographic coordinate system, WGS84 as per surmise.

The other data with the -62.5 central meridian is a projected coordinate system, but you haven't given the full description.  I suspect that it is a 3 deg MTM projection of some kind that is just named with the St.Kitts and Nevis projection... leave it.  The decimal degree data will project on the fly to match the other layers in the data frame..  Since you seem to be relatively new to this, you can use the Project Tool in ArcToolbox (Data Management, Projections and Transformations, Project) and 'project it' to a new file, using the projection of one of you 'layers' when given the option.  Then you can remove the original data and work with data in that projected coordinate system.  Under no circumstances change an already defined projection, or define one incorrectly.

Things can go bad real quick.

[NigelBardoe]

Thank you.    I had originally tried to use the project tool but as it was a custom projection, ArcMap did not recognize it and could not transform it to anything else.   Most of my reason for thinking the projection was so strange was the fact that it was so off from my added data which had the values in the wrong field and I had no other control data to compare it to.    Thanks again for all the help, I will count this as answered.