Hi,
I'm sorry that I didn't reply earlier.
Note: Pro is different; will explain below after ArcMap info.
ArcMap will reproject all layers that have coordinate systems that aren't the same as the data frame's. What it won't do is the geographic/datum transformation part if there are no transformations already set. Let me show the workflow.
GCS = geographic coordinate system (datum); PCS = projected coordinate system
Data frame in PCS2 based on GCS2
Layer in PCS1 based on GCS1
- PCS1 is unprojected to GCS1
- GCS1 is transformed to GCS2 (if transformation is set, otherwise it's internally redefined to GCS2)
- GCS2 is projected to PCS2
If a layer is in GCS1 already, then just steps 2 and 3 occur.
To check on whether there are transformations set,
- Open the data frame properties (right click Layers, then select Properties)
- Click the Coordinate System tab
- Click the Transformations button
- In the top box is the unique GCS in the layers and data frame
- Below that is the data frame's GCS
- At bottom is a pull-down that may show the name of a transformation or "None".
- Select a GCS in the top box, then check the pull-down to see if a transformation is set or not.
- We show a sorted list based on extent and accuracy, but it can be fooled by having a world base map, etc
How much data can be off depends on the two GCS involved. Two newer GCS like WGS84 and ETRS89 should agree within a meter or so (or better). British National Grid uses OSGB 1936 so it will differ by more like a hundred meters from ETRS89 or WGS84.
Pro: Uses a lookup table to automatically set the geographic transformations. The lookup table usually uses the most generic transformations available which sometimes are not the best transformation for a particular area. Just as in ArcMap, you can open the properties of a map and change the transformation.
Melita