State Plane to WGS84

Question

Hey all!

I am running into the all to famous conundrums of projecting State Plane to WGS84.

I have eastings and northings point coordinates supplied by a contractor for important infrastructure, so accuracy is very important. Seemingly the data is so close, but no matter what I seem to do, all of my points are almost exactly a foot off (at least they are consistently off!).

I start by adjusting the coordinate system of the map to the State Plane projection for my area.

I then add my data coordinates, which are on a spreadsheet. I then use 'XY table to point' to create my initial State Plane projected points.

After, I have the points added, I use the 'project tool', but the WGS84 points are consistently one foot off.

I have tried inputting the vertical datum (NAVD88 is what was used for the original data... I've been curious if vertical and horizontal having different measurement types is what is impacting my results?) in both the project map and within the project tool, but keep getting similar results.

Any help is very appreciated! Thanks for the guidance!

Jacob_K · Answer

Hey @BryanRichards!

Does trying EPSG code 6428 work?

This is for Colorado North, NAD83(2011), US survey feet. This would be the more recent system, compared to the one you're currently using.

BryanRichards · Answer

Here is an example of my coordinates.

This is what the as built say about its projection: . ALL COORDINATE INFORMATION SHOWN IS BASED UPON STATE PLANE COORDINATES, COLORADO NORTH ZONE USING THE CITY OF STEAMBOAT SPRINGS GIS CONTROL NETWORK. VERTICAL DATUM IS NAVD88.

Knowing that, I used the code 2231.

Thanks for the help!

Jacob_K · Answer

Hi Brian!

This can be an issue due to a few reasons. Is there any chance you can provide an example pair of coordinates before and after projection?

Do you know the exact coordinate reference system used by the contractor? If possible, the EPSG code (or the full coordinate system name) would be very helpful. For example, different realizations of NAD83 (such as NAD83 vs. NAD83(2011)) can introduce small but noticeable shifts.

I'd also verify the units. State Plane coordinate systems can be defined in US survey feet, international feet, or meters, depending on the state and jurisdiction. A mismatch between the units used by the contractor and those assumed by the projection can produce consistent offsets.

I think simple rounding is less likely if you're seeing a consistent offset of about a foot, but it's still worth confirming that the original coordinates haven't been truncated or rounded.

I'm not as concerned with the vertical coordinate system; NAVD88 only affects elevations and shouldn't impact the position during a typical projection unless you're performing a full 3D transformation.