In some circumstances, it may be necessary to clear the coordinate system entirely for a map by setting the coordinate system to Unknown. When a map is in an unknown coordinate system, the application skips operations that are usually performed automatically to correctly position and align layers in a map.
Operations that are not performed when the coordinate system is unknown include the following:
| Operation | Behavior when the coordinate system is unknown |
|---|---|
Projection | Layers are not projected on the fly. |
Datum transformation | Layers ignore datum transformations specified in the map. |
Unit scaling | Values are unitless. For example, a layer with an extent of 10 decimal degrees draws the same size as a layer with an extent of 10 meters. |
Origin offset | Layers draw as if the origin of their coordinate system is 0,0 even if it isn't. For example, a layer in a UTM 10N coordinate system (which has a central meridian of -123 degrees) draws in an unknown coordinate system as if the central meridian is 0 degrees. |
Because layers draw without these operations in a map with an unknown coordinate system, you can use an unknown coordinate system to visually compare or otherwise troubleshoot the spatial references of two or more layers.
While unknown coordinate systems can provide information about the state of a layer's spatial reference, they are not suitable for creating, editing, measuring, or analyzing data within that layer. The most prominent limitations of using an unknown coordinate system include the following:
- Editing is not permitted.
- Some navigation and analysis operations and tools are not available. Others are likely to produce unexpected or unreliable results.
- The map scale is based on unitless coordinates.
- The Measure tool measures unitless, planar quantities only.
- Vertical coordinate systems cannot be specified.
- The map cannot be converted to a local or global scene. Existing scenes cannot be set to an unknown coordinate system.
When you import an ArcMap document (.mxd) with an unknown coordinate system into ArcGIS Pro, the resulting map has an unknown coordinate system.
Follow these steps to define an unknown coordinate system for a map:
- In the Contents pane, right-click the map or scene and click Properties.
- On the Map Properties dialog box, click the Coordinate Systems tab.
- From the Coordinate Systems Available list, choose
<Unknown>. - Click Apply to set the coordinate system, or click OK to apply the settings and close the Map Properties dialog box.
If a vertical coordinate system is set on the map, it is removed when you apply the change.
Dive-in:
Although the values are unitless in an unknown coordinate system, a unit framework must be used internally to draw features to the screen. Unknown coordinate systems use unknown units, identified by unk after each value. For the purpose of defining map scale, these unknown units are treated as though they are equivalent to meters. Be aware that these units cannot be considered true meters in the real world, so no measurements or other calculations should be performed based on them or the map scale.