Convert an Athens Metro OMA coordinate

Pick the WGS84 frame on purpose. Only the real-world one belongs on a map or in a GNSS receiver.

Open Calculations > Survey Calculator > OMA L4 (Athens Metro).

The calculator converts between the local grid of Athens Metro Line 4 (project CON-005/21, Alsos Veikou–Goudi), the Greek grid EGSA'87 (EPSG:2100), and WGS84. Grid-to-grid conversions were checked against the project's published network at the millimetre level. The parameters are for that project. Do not reuse them on another site.

Three things called WGS84

On Line 4, three frames are all called WGS84. They disagree by up to about 200 m. The same pillar, T10, in the manual:

project frame:   37.99985584 N   23.78717579 E
TBC global:      38.00232381 N   23.78844686 E
REAL WORLD:      38.00093975 N   23.78691125 E

Project WGS84 is the OMA grid projected backwards. It is about 123 m from the ground. This is what the project tables, and TBC, print as WGS84. Use it only to compare with a project deliverable.
TBC global grid applies the project's 7-parameter Helmert on top of that. It is about 205 m from the ground. Use it only to reproduce a TBC printout.
Real-world WGS84 goes OMA → EGSA'87 → the EPSG:1272 shift. It is about 0.9 m from the network's own GNSS positions. This is the frame for a map or a receiver.

KML export and the Google Maps links stay disabled unless the direction is the real-world one.

Directions

OMA → EGSA'87 is a 2D similarity, about 2.6 mm RMS and 5.3 mm maximum against the published table. The reverse is the exact inverse. OMA → real-world WGS84 and the reverse close a round trip to under 5 mm.

Use the direction dropdown. Do not paste a project-frame latitude into a receiver and expect it to land on the pillar.