Why does Lat/Long differ depending on the coordinate system of the data?

Question

I obtained ETRS89 (ETRS_1989_ETRS-TM32;WKID: 3044 Authority: EPSG) data and I am using a Gauss-Krüger coordinate system (DHDN_3_Degree_Gauss_Zone_3;WKID: 31467 Authority: EPSG). Now I need to calculate the WGS84 Lat/Long coordinates. Using ArcGIS for Desktopts field calculator, I have the option to use the "Calculate Geometry" tool depending on the coordinate system of the data source (ETRS89) or the data frame (Gauss-Krüger). Depending on the choice, values differ by the third digit, which is an offset of approximately 200 m. I use the right transformation (DHDN_To_ETRS_1989_8_NTv2), the data is shown on the correct position, but the Lat/Long output using Gauss-Krüger is wrong. When I load the Lat/Long data again it is not on the correct position anymore. I am trying to understand that issue and also to solve that, if possible.

Answer 1

You are using the wrong transformation as you are having actually 3 reference systems.

You want to have WGS84 coordinates. To obtain those you set the you dataframe to WGS and calculate the geometry based on the dataframe. I guess you forgot to set the transformation here to DHDN_to_WGS84. You only set transformation DHDN_to_ETRS for your two layers. Arcmap won´t use it for your third transformation to WGS84. Thats why you get the typical 180-200m transformation offset.