Getting real-world coordinates from a paper map with a custom projection

[NatalieSusmann1]

I have a paper map with survey points. They used a custom graticule and origin, and included a site register which lists each site's location according to this system. This was all done to preserve data integrity. My end goal is to visit these locations in person/view them in Google Earth. I know the original projection of the map with which they recorded locations, and also the coordinates of their origin point.

While georeferencing is an option, I am wondering if there is some way to use Excel, etc. to convert these coordinates (i.e. 117 N 140 E) into usable coordinates (WGS decimal degrees for example).

I was thinking about making a custom grid, digitizing the points by hand based on the grid locations, and then reprojecting the points. Would this work?

[DarrenWiens2]

I know the original projection of the map with which they recorded locations, and also the coordinates of their origin point.

If I understand correctly, you can create a custom coordinate system using the original projection as a starting point, and change the false easting and northing parameters to account for the offset.

Once you've got the points entered and defined by your custom projection, you can Project the points to whatever standard coordinate reference system you'd like.

[Robert_LeClair]

Depending on how the original numbers are represented, the field calculations would be pretty easy as documented here - Converting degrees-minutes-seconds values to decimal Ddegree values—Help | ArcGIS for Desktop

From there, you could add it as an XY Event Layer in ArcMap and export it to a new feature class.

[NatalieSusmann1]

So these locations are in Greece. One location has the coordinates of 117N, 140 E, for example. I figured out the 0,0 to be 413829.262, 4122499.713 meters, using the Greek_Grid GCS.

[DarrenWiens2]

You need more information than what you've provided so far. For example, were the original points collected according to the Greek_GCS? What projection, if any? What are the units for 117N and 140E (degrees, metres, etc.)?

Depending on some of these answers, you may be able to just add a certain amount of degrees to each coordinate, or it may be a very convoluted formula.

[NatalieSusmann1]

The locations were recorded in 1978 by hand using aerial photographs, and then were transferred by hand onto a 1:5000 topo map, both made by the Greek Army Service. These maps use the Hellenic Geodetic Reference System: Hellenic Geodetic Reference System 1987 - Wikipedia, the free encyclopedia

These original maps are not available.

On their published site map, they use a 200X210 (unknown units) grid. The scale bar measures 4 km, and each 50 grid mark roughly equals 6.25 km. The grid units correspond to the coordinates given for each site.

[MelitaKennedy]

Unfortunately, they didn't use GGRS87; it was defined after 1978. The map may have used the older Greek Grid, which uses a map projection that isn't supported in ArcGIS.