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
Thank you for your response, very intense, nice job! I believe what you are saying is the current v8i design file contains a Global Origin at 0,0,0 and an additional origin assigned to a projected map by the datum. The auxiliary origin does not interest me, just the relationship between the design file global origin and the projection map origin being used. It sounds like there are potentially two origins (global origin and map origin) and if there is no projection assigned, then only the global origin exists in the design file?
As it's the middle of the night where Lorys is I'll answer your last question. Yes only the global origin exists by default. It is possible to reassign the co-ordinates of the GO from 0,0,0 using the GO= keyin. We do this in order to use real world (in our case UK Ordnance Survey coordinates) but keep the model close to the origin. In Connect edition (the current Microstation) you can also assign a geographic coords system using the coordinate system button on the Utilities tab.
If I am using a State Plane CRS here in North America. Is it correct to assume the map origin is set based on the projection, which does not use the global origin directly, but the global origin remains at 0,0,0. So that every map projection will have a unique position relative to the global origin at 0,0,0, as managed by Microstation?
At the edge of my knowledge on this but yes. I believe you are correct.