By default, a map or scene is assigned the coordinate system of the first feature layer you add to it. Subsequent feature layers with different coordinate systems are projected to the map’s spatial reference (on the fly) in real time.
A geographic datum transformation projects the data in the current map or scene without changing the source data. By default, a best transformation is assigned based on its accuracy and the extents of the data. You can view or specify the transformation on the Map Properties dialog box on the Transformation tab.
When you edit feature layers, consider the following:
- Editing feature layers that are reprojected on the fly can introduce spatial errors into data, degrade map performance, and take more time to draw features.
- On the Edit tab, in the Manage Features group, click the Editing Status button . Layers that are projected on the fly appear with this information message.
- When you choose a spatial reference for editing, choose a projection that best preserves the geometric properties that are most important for your work. All projected coordinate systems introduce varying levels of distortion to the shape, distance, direction, and area of feature data.
Editing feature data that is reprojected on the fly can introduce spatial errors into data and is not recommended. The magnitude of errors increases the farther your edits are outside the optimal region of accuracy for a given projected coordinate system.
Typical errors include unexpected shape distortions, misaligned extended or trimmed segments, and snapping irregularities. Visual inconsistencies can also appear between unfinished geometry, the pointer, and rubber band feedback as you create or modify features.
To avoid these errors, assign the map a coordinate system with an area of use that is appropriate for the geographic location where you perform your edits and one with spatial properties close to the data source properties.
The best practice is to specify a coordinate system for the map or scene that matches the feature layer source data you are editing. If you need to reproject the source data instead, consider using the Project geoprocessing tool.
Geodatabase topology is validated at the data source. Fixing geodatabase topology errors while a feature layer is projected on the fly can cause previously fixed errors to reappear.
The best practice when fixing geodatabase topology errors is to edit the features using the same coordinate system as the data source.