Hi, I have a loose understanding of projections, and I am trying to figure out from a CAD perspective how projections are defined.
For example, when someone sends me CAD data it has no projection defined, I convert it from a DWG to a shapefile, and then since I know roughly where it is I just try to define the projection with a few guesses ie. UTM zone 11 in meters, and it works out.
What I want to find out is how the projection is defined when the data is first drawn in CAD (I have never used CAD and don't know anything about it really). Ie. When an engineer sits down to draw something, how do they determine that it is in UTM zone zone 20 and not MTM zone 6 or something.
The reason I need to know is that I can't figure out the projection of a certain DWG and I am trying to relate to an engineer what information they can give me to help me define the projection
Projections in CAD are defined in the Drawing Settings - Unit and Zone window. The available coordinate systems should be the same as to those available in ARCMAP. For example NAD83 ****, in either projected or geographic coordinate systems. The coordinate system parameters would match those as defined in the EPSG (European Petroleum Survey Group) files. That being said, the chosen coordinate system for CAD drawings is often No Datum, No Projection (meaning the coordinate system is not formally defined). It may simply be because the engineer forgot to assign the coordinate system prior to saving it. This is typically the case when coordinate systems are easily identified when importing into ARCMAP. Unfortunately, it may also be that a random coordinate system not tied to any known geographic coordinate system was utilized. When the latter occurs, it is unpredictable where the drawing will be positioned in ARCMAP. Many local coordinate sytems have been developed that are not correlated to known geographic coordinate systems.
The minimum information from the engineer should include the local coordinate system's point of origin, orientation, scale factor(if any), and units. Once these are known, projection parameters can be created.
Projections in CAD are defined in the Drawing Settings - Unit and Zone window. The available coordinate systems should be the same as to those available in ARCMAP. For example NAD83 ****, in either projected or geographic coordinate systems. The coordinate system parameters would match those as defined in the EPSG (European Petroleum Survey Group) files. That being said, the chosen coordinate system for CAD drawings is often No Datum, No Projection (meaning the coordinate system is not formally defined). It may simply be because the engineer forgot to assign the coordinate system prior to saving it. This is typically the case when coordinate systems are easily identified when importing into ARCMAP. Unfortunately, it may also be that a random coordinate system not tied to any known geographic coordinate system was utilized. When the latter occurs, it is unpredictable where the drawing will be positioned in ARCMAP. Many local coordinate sytems have been developed that are not correlated to known geographic coordinate systems.
The minimum information from the engineer should include the local coordinate system's point of origin, orientation, scale factor(if any), and units. Once these are known, projection parameters can be created.