Project an unknown coordinate system on a map

I am currently working with a data set, which uses an unknown coordinate system from the south island of New Zealand, using x and y coordinates. I have these in an Excel sheet, but no further information on the coordinate system is known.

I have used Excel calculations and GPS approximations from Google Maps to translate the unknown coordinate system to the NZTM one, but the results still show a deviation of about 5km for some points. As I need to have very precise positions, I am thinking about using ArcMap to project the data points on a New Zealand map, adjust the points so it fits to the map and then use some kind of translation to get from the data points to e.g. GPS coordinates.

I've tried to illustrate that with the pictures bellow.

Picture one: The blue dots represent the x,y coordinates of the unknown coordinate system. Is it possible to insert those data points into ArcMap like shown on the picture?

Picture two: The second step would be to adjust the data points s.t. it fits the New Zealand map. Is such an operation possible with ArcMap?

The last thing would be to use exchange the initial x,y coordinates with e.g. GPS coordinates.

2 Answers

Additional information would be useful here. How do you know the coordinate system is unknown? Is there any info in the metadata?

If there is no coordinate system projection assigned to the file, you can define one. Sometimes people forget to do this (sometimes it's just wrong). Your goal is to guess the correct projection. Make a copy of the file and use the Define Projection tool if one is not in the metadata. Try a geographic coordinate system first (WGS 1984 World) or whatever local projection makes sense. Look at other files from the area to see common local projections.

Make sure to do this in a completely new map window with no other layers. Do not just remove the layers, but start with a fresh map because the ArcMap viewer will use the projection of the initial layer you add to show all the data.

You can also try the tool Project, which is used to transform one known projection to another known. Only use this once you are sure what projection you are working with. Let me know how it goes. Also, there is a georeferencng tool in Arc where you can match up known areas and create your own projection, but this can create many errors. I can expound on this if needed.

• Hi Rachel, Thanks for the support. I definately don't know which coordinate system I am working with. It looks like it's completely self-made and that's why I can't transfer it to any other systems. I have check several international and New Zealand systems and it does not resemble anything known. Therefore, I want to use the approach by overlaying the coordinates with a Map and translate the coordinate system this way. Jun 16, 2019 at 16:33
• so, nothing in the meta data at all? What are the values of your coordinates? Maybe you can share the file? If so, I'll take a look. Also, you can georeference the file. You'll need to strip any coordinate system defined in the file. Here is a tutorial to georeference - youtube.com/watch?v=PHtxbpboDro. There are others out there too. Jun 16, 2019 at 17:08
• I can't share the file, but the coordinates do look like this: x:7470575 y: 46454555. I think the georeferencing tool does only work with a picture and it seems like it does not help me to convert the coordinates. Jun 17, 2019 at 17:03
• Still thinking you just need to put the right name on the projection so Arc knows how to warp it. Your units are in meters. Try to define a common UTM projection for New Zealand. Links to official NZ projections - linz.govt.nz/data/geodetic-system/…. For georeferencing, you line up your points with an image of a known projection, and then define your file with the same projection -youtube.com/watch?v=xpg_X-7tHss. Use a public folder in a free dropbox acct. to share the file. Offer still open. Jun 19, 2019 at 12:38
• Also, where did you get your file from? Jun 19, 2019 at 12:52

I found the solution for the issue with another question asked here:

Georeferencing Data Points?