An Esri coordinate system definition must be assigned to a .dwg file to properly position GIS data within the drawing and to ensure ArcGIS web layers added to the drawing will be positioned correctly.
Once assigned, an Esri coordinate system definition is stored within the .dwg file. When an Esri coordinate system definition is assigned to a drawing, all subsequent data added to the drawing by ArcGIS desktop or from ArcGIS map, image, or feature services will be properly positioned within the drawing to the specified coordinate system regardless of the coordinate system of the source data. Furthermore, when the drawing saved with and Esri coordinate system assigned is used as input to ArcGIS desktop, it will be properly positioned.
If no Esri coordinate system is defined within the drawing, the coordinate system of the first web service added to the drawing will be used to define the drawing's coordinate system. This coordinate system may or may not be appropriate for your drawing. It is strongly recommended that you always assign the coordinate system of your drawing before adding ArcGIS data to it.
Note:
ArcGIS for AutoCAD requires its own coordinate system information distinct and separate from any AutoCAD coordinate system or geolocation information.
Coordinate systems
The coordinate systems that can be used with ArcGIS for AutoCAD include the following.
Geographic coordinate system
A geographic coordinate system (GCS) is a system of latitude and longitude that uses a three-dimensional spherical surface to define locations on the earth. A point is referenced by its longitude and latitude values. Longitude and latitude are angles measured from the earth's center to a point on the earth's surface. The angles often are measured in degrees (or in grads).
Projected coordinate system
A projected coordinate system (PCS) is defined on a flat, two-dimensional surface. Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions. A projected coordinate system is based on the Cartesian grid. It uses linear units of measure, such as meters or feet, for coordinates so that calculations of distance and area can be performed with those same units.
Custom coordinate system
Create a custom coordinate system when CAD drawings are created using nonstandard units of measure and a suitable coordinate system cannot be found. The flexibility of a custom coordinate system allows you to modify linear units of a projection file to adjust the position of the data in either the east-west or north-south direction to align data.
For instance, adjusting a false easting value in the projection file adjusts the position of the data to the west; that is, it moves the data to the left. Adjusting a false northing value, such as assigning a larger value, moves the data to the south. Additionally, CAD files are often rotated to fit the drawing in its model space, which requires removing the rotation from the source file. Another solution is to create a custom projection using azimuth or rotation parameter to align the CAD data.
Geodata coordinate system
Geodata coordinate system definitions established in standard AutoCAD or Civil 3D are not used by ArcGIS for AutoCAD. However, geodata coordinate system definitions referenced by an ESPG number can be used to assign the Esri coordinate system definition for that same ESPG number. Use the import geodata coordinate system definition option on the Coordinate System Selector user interface or the Import option of the -esri_coordinatesystem command.
Geographic datum transformations
After assigning the coordinate system you want to use within your drawing data, you may still want to use web layers in a coordinate system that requires more information to handle differences in the spatial reference between coordinate systems. The science of mapping a flat map to a spheroidal Earth provides various options to generate the best outcomes. In most cases, the standard transformation used to automatically project data from the coordinate systems of your web layers into your drawing's coordinate system is sufficient. However, sometimes you will need a specific datum transformation to convert data between different geographic coordinate systems more accurately. Unless your data lines up, you'll encounter difficulties and inaccuracies in any analysis and mapping you perform on the mismatched data.
A geographic datum transformation is a calculation used to convert between two geographic coordinate systems to ensure that data is properly aligned within a map.
The best geographic coordinate system to use depends on where and how much of the earth's geography you are mapping. In ArcGIS for AutoCAD you can add a datum transformation to the drawing using the provided AutoLISP routine, esri_coordsys_DatumTransformations_set, and it will be applied when necessary. This is a more advanced feature and typically you will know or will be instructed as to which transformations you will need to specify by your GIS administrator.
Note:
Geographic datum transformation are supported in the ArcGIS system for ArcGIS map layers and feature layers, but they are not supported for ArcGIS locators or image layers.
See also
For more information, see the following: