If you have collected data from a variety of sources, chances are that not all layers contain the same coordinate system information. The coordinate system of a data frame in ArcMap can be different from the native coordinate system of the data sources represented by the layers shown in the data frame. In this case, ArcMap projects (on the fly) the features in these layers to the data frame’s coordinate system. ArcMap also lets you edit features while they are projected. If you start editing and any of the layers in the database or folder that you plan to edit are in a different coordinate system from the data frame, you receive an Choose not to continue editing if you want to change the coordinate system used by the data frame to match the native coordinate system of the layer or layers you want to edit. When a projected feature is edited, all edits occur in the data frame’s coordinate space and are projected back to the feature’s native coordinate system as the feature is stored. For most editing operations and most coordinate system transformations, the integrity and accuracy of the feature is maintained. However, it is important to note that certain editing operations may produce unexpected alignment or accuracy problems, depending on the coordinate systems being used. Specific editing operations that may cause issues include changing the shapes of features, snapping to the edge or boundary of features, or extending and trimming features. These problems are more likely to occur when the features you are editing are close to the edge or beyond the area of use of the coordinate system. It is important to understand that coordinate systems have an area of use. The area of use defines where it is appropriate to use a particular coordinate system. You need to ensure that your data is located within these limits. Another reason problems occur is due to the distortions that affect features covering a large geographic extent. Features that are stored in layers with unknown coordinate systems cannot be projected by ArcMap. Regardless of the current data frame coordinate system, any edits made to these features occur in the native coordinate system. Beyond these issues, mismatched coordinate systems between the data frame and layers in that data frame may cause certain edits to fail because of differences in tolerances. Setting the coordinate system of the data frame to match a layer resets the data frame’s tolerance to match the layer. To do this, stop editing, then follow the instructions in Extending lines is a common case where you may see unexpected results when you are editing data that is being projected on the fly. For example, while editing in a different projection, you extend a dangling line to make it connect to another line. While the lines appear to be snapped together in this projection, you may find that the line has been extended too far (or not far enough) when you display the line again in the native coordinate system. In another example, a new point is added on the state border between New York and Pennsylvania by snapping the point to the boundary. To illustrate the problem, the point is added in the native coordinate system (left) and again when the features are projected to a different coordinate system (right). As the point in the projected coordinate system is created, it is projected to a location in its native coordinate system. The graphic below shows both points that were added. The green square represents the point feature that was added in the native coordinate system, and the red circle represents the one added in a different coordinate system. Even though the red circle was snapped to the straight line between the two states, due to distortions caused by projection, the feature will never be exactly on the border and is approximately 500 meters from the border. You should fix topology errors in the layers’ native coordinate system by making sure that the coordinate system of the data frame is the same as the one used by the layers you are editing. Fixing errors in projected coordinate space could result in a recursive problem of using topology to fix an error, validating the results of the fix, then discovering that the error reappears. This is not a problem with the fix that you apply, rather, it is due to inaccuracies introduced when the feature is projected back to the native coordinate system. For detailed product documentation and white papers on supported coordinate systems and geographic transformations, visit: Changing the coordinate system of a data frame or reviewing the coordinate system of a layer that appears in the Start Editing information message Source.