November 16, 2007

Fast Break Projection On-the-Fly

Basketball practice has started for the Spirit Preparatory Academy Lady Warriors, and this year we coaches are focusing on our team’s fast break. In basketball if you can run a successful fast break offense all you have to do to score an easy basket is to have one-more of your players run down the floor faster than that of the opponent’s. Similar to the adage that when being chased by a bear, I don’t have to run faster than the bear; I just have to run faster than you!

Running the fast break in basketball greatly simplifies the game, but requires that my team be organized and focused on the goal of moving with purpose down the floor whenever our team has the ball. This style of basketball requires preparation in advance of both structure and conditioning. If our team has poor positioning, or spacing running down the floor, as to allow one defender to defend two players because of their proximity, or if our players don’t have the physical stamina to repeatedly run down the floor, the fast break doesn’t work.

When I create Microstation and AutoCAD drawings from GIS data many times I am supplying design reference and context for new design work that will be incorporated back into the GIS basemap latter in a project’s life cycle. There can be a practical need to work in different coordinate systems for local design work verses the storage of regional basemap data. Or, in some cases the need to work in different coordinate systems becomes a matter of an organization’s arbitrary preference for one or another coordinate system based on any, or no reason at all. I can change the coordinates of the CAD data I create from GIS data using the ArcInfo EXPORT TO CAD tool in ArcGIS 9.2 service pack 4. Starting with the ArcGIS 9.2 Service Pack 4 release, the EXPORT TO CAD tool will honor the coordinate system defined in the projection file (.PRJ) of the target output CAD file. This means GIS data stored in a UTM meter’s coordinate system can be output to a Microstation design file where the .PRJ file designates a State Plane coordinate system in feet, and the CAD elements being created from the GIS features in UTM will be generated in the correct State Plane location in feet within the Microstation design file.

When using the ArcInfo level licensed EXPORT TO CAD geoprocessing tool starting with ArcGIS 9.2 Service Pack 4, If no .PRJ file already exists for the target or seed CAD file the coordinate system of the input data will be used as for the coordinate system of the output CAD entities, and a companion .PRJ file will be generated for the CAD file automatically.

For Microstation design files it is necessary to set the design cube/plane coordinates definition to ensure proper behavior. I need to ensure my Microstation design plane/cube and its units are adequate set to store the GIS information in the .DGN file. To trigger the new functionality of the EXPORT TO CAD tool I need to identify which coordinate system the coordinates in the Microstation file will represent. I do this using a companion coordinate system definition file .PRJ.

The first of these two steps to define the units and design cube/plane is a purely a Microstation process usually set as part of the Microstation seed file creation. This process is a purely mathematical requirement of how Microstation will be used to record your map as a file of vector geometry. The other process, the creation of a .PRJ file, is specific to ArcGIS which identifies the coordinate system and how the information in a CAD file relates to our round Earth. Just like the execution of the fast break in basketball preparation early in the season makes the players work together more smoothly, and enables success in the game.

AutoCAD .DWG files don’t have the concept of a design cube/plane, but do use the same .PRJ companion file strategy in ArcGIS for identifying the Cartesian coordinates contained in an AutoCAD file and how they relate to the round Earth’s spheroid. The same information stored in the .PRJ companion file can also be included inside the .DWG file according to the techniques starting to be used in ArcGIS for AutoCAD.

Consult you Microstation documentation for the proper creation of coordinate definitions and units for your Microstation Design files and seed files, and use the ArcGIS desktop or online help in the creation of .PRJ files.


Blogger Luke said...

Hi Don -
I'm working with a CAD map in ARCGIS 9.2 and I've found many of your suggestions to be very helpful! I'm trying to create a reference grid tying it in to GPS points that I've overlayed onto the CAD map - I've had no success using the grid creation options with ARCGIS - any suggestions?

5:28 PM  
Blogger Don Kuehne said...

I am not sure I fully understand your question. Tying the coordinates of a CAD file to a Map in ArcGIS is done most easily using the georeferencing toolbar in ArcMap. The tool performs a two point affine transformation: Move, Scale and Rotate. Error distributoins, rubbersheeting and the alike can only be performed on data that is in the GIS format. You can copy the CAD data to a geodatabase and rubbersheet the dat there. Hope that helps... tell me more if I missed it.

9:34 AM  

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