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Design Plane Origin


I am reading a Bentley white paper titled "US Coordinate Systems in MicroStation v8i & Bentley Map v8i."  At the top of page 5, it states "A Microstation design plane itself is a Cartesian coordinate system with the XY origin at the center of the design plane, except when the global origin has been shifted."  I assume that the origin of the design plane corresponds to the projected origin of the map.  However, I question if there could be an additional origin introduced to the design file; a global origin, and a map origin?

  • A lay GIS understanding for msnt

    in v8i the global origin doesnt really do anything to the projection, its only if you save it  back to prev8i that the GO will change things...

    Prior to v8i we had a  limited design plane 2^64 in x, y and z direction master units ( v8i basically has no limit unless you mapping  galaxies) so for some of us say here in australia, victoria  we could not  fit the entire state plane in meters and have the full range of our state datum coordinates, so in order to fix this and not end up with negative coordinates we created a False northing and set the go in the middle of the state plane... only problem was with referencing everyone had to use the same settings for the altered  GO in all their dgns or maps wouldnt line up, later when we were able to do datum transformations through other GIS type software preBentley map we just told the package what we wanted and it  remapped and datum trans formed...or allowed surveyors to do the transformation shifting

    Today the global origin really is 0,0,0 and references are related by true  coordinated to where the real 0,0,0 is so things land ontop of eachother at the true coordinates, added to this we set the datum or gcs, geo coordiante system, this allows xrefs of different CGS to reproject on the fly...

    The projection map origin is set by the CGS , ellipse and interrnational GEO mapping org setting or code this is all done via clever mathemantics and comes up with a true mapping  origin which we never change , touch or modify... msnt  knows what to do if you just define it and the code is in the header so most  GIS software can read it and determine the projection and datum and make the relative adjustments to display  or reproject...

    There is also the ACS or auxillary coordiante system this allows you to make something relative to a new origin or temp origin, ie move 0,0,0 to the corner of a building and rotate the world so left and right are 0, and 180 degrees, architechs love to draw this way but the tend to move the world to 0,0 rather than move 0,0 to an arbitary corner.... when I get drawings or maps done correctly with the acs changed I just reset it  back to 0,0 and maps now realign with real survey again....the acs does make  drawing layouts easier but will drive surveyor mad as nothing is in true coordinates only relative local  but done right can easily  be reset to true..

    So in  a nut shell dont worry about global origins and map origins  just use the seed files from bentley  set your CGS  in youre seed file and save it with the CGSnameas part of the seed name do this only with the most popular one you use  but remember you may need to tell mstn what the cgs in a dgn is  if its not the one you always use and let it trans form .. this is very important  for xref of survey or map data from differnt datums and sources... some data is only available in WGS84  Long/ Lat and your using  Eastings and Northings so msnt  should notice this and transform or reproject the xref to align with the active drg cgs...

    The only time the GO will be a problem is if  you a have to export the dgn from V8i hardly anybody  uses the V7 format

    Then you will need to make a copy of you v8i file change the GO to the V7 settings then export the copy v8i for to V7 and it will keep the changed go so other drgns of the the same go will overlay coordinate survey correctly and give the right xy values to coordiantes...

    While not 100% correct from a GIS mapping perspective all the points above will help with getting things  correct automatically as much as possible

    Hope I havent  waffled on too much and was informative and instructional.


    MicroStation user since 1990 Melbourne Australia.
    click link to PM me 

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