Design Plane 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.

Lorys,

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.

At the edge of my knowledge on this but yes. I believe you are correct.

Ok, thanks again for that feedback.  So if you adjust the global origin, then that is how to influence the coordinates showing in design file, since Microstation is reading from 0,0,0.

You *can* adjust the Global Origin and it *will* influence the coordinate readouts.You must make a distinction however. IF you are doing GIS/Mapping type work with a GCS (like a State Plane projection), then you have no reason to mess with the Global Origin. You assign a GCS to the file model and off you go. Without a defined GCS, then you're resorting to try and imitate one using an Auxiliary Coordinate System and/or twiddling with the global origin (not really recommended).

Yes, good answer. So there is just the one origin, unless you use an auxiliary. The GCS simply changes the way the coordinates are interpreted. If you have a drawing with stuff in it and you assign a GCS, it will ask if you want the elements re-projected or if they are correct as drawn. GCS is a real nice feature for anything you're drawing in real world positions. It allows interaction with things like Google Earth.

Thanks Bob for your feedback.  My question is more academic in nature, trying to understand the relationships between the origin(s) involved with files associated with coordinate reference frames.  It makes sense that a projected map origin would be different than the global origin (do not coincide).  Although I would be interested to learn what you  mean when you say "GCS simply changes the way the coordinates are interpreted"?  It seems to me that MicroStation always reads coordinate values from 0,0,0, and any coordinate correct data is simply positioned, scaled, rotated, in relation to the global origin, but the coordinate correct data also has its own origin?

All cords are read from the go 0,0 then the algorithms for the CGS datums redisplays / moves the data to the true coodinates for that  datum... in the old days the surveyor would survey all the points  relative to established markers then back in the office use complex calculations to transform all the points to the desired datum, now days the total stations and gps does all this for them... but in mstn we now can do the transformations just like we were using a GIS package which essentially does the same  ie read the existing datum and origin and either show as is or reproject/ transform to new datum... before we had this in mstn I needed to import  the survey dgn in our old state plane datum with MapInfo then transform it to new state plane and export the new map to DGN ... now its a one stop shop with mstn so its the GCS software inside msnt that  does all GIS transform stuff so you don't need to know anything about GO  and map origins , ellipse setting etc as its already set for you from pull downs in the GCS tools... I love it! As you can no doubt tell.. only time you need to worry is when there isn't a GCS datum available in the list... then its a lot of work to find a compatable one that works close enough  or you have to get Bentley  Map expert to build one to import so you can keep using it over and over... not easy to get this... so we just use  GIS like QGIS or mapinfo to do it instead.. but rare now days   for most countries