Monday, July 10, 2017

Revit 2018 - GEO Reference and Shared Coordinates

I replied to a thread at RFO that asked about Revit 2018 touting support for AutoCAD's GEO Reference feature.

On the surface, there is no obvious difference between how things worked in 2017 (or older versions) compared with 2018. Over the years you may have noticed that the Location Dialog, the one that allows you use a map to locate your project did not do anything at all related to the Shared Coordinate system. All that action did was provide a way for Revit to; originally calculate sun position (and therefore shadows) more believably and more recently to allow for energy analysis estimation to be done. Revit 2018, assuming the source DWG file is using AutoCAD's GEO Referencing feature, it is possible for Revit to inherit this data to affect not only the Location (Sun and Energy Analysis) but also the coordinate location of the project (Shared Coordinates).

The thread at RFO also asks about the 20 mile threshold Revit has regarding model size and warning us about model accuracy. The following is a restatement of things I've written in the past. Specifically they asked if there was any change to this in 2018. There isn't that I know of. I included the following to superficially explain the reason it exists.

The 20 mile threshold is a math and computer science problem that Revit developers choose not to lie to us about. They want us to keep the model as close to the file's mathematical origin as possible. External files (and internal modelling) that have data whose extents are larger than 20 miles begin to influence the accuracy of the calculations required to generate and display the model faithfully.

More often than not a civil file is not really larger than 20 miles. It just has elements that are farther away from the origin than that. Revit doesn't mind that issue and it doesn't mind assigning very large coordinates values to the shared coordinate origin (Survey Point).

It only cares when there are elements that are beyond the threshold. For example a file that only has two short line segments that are 30 miles apart will cause a warning. A file with an entire set of contour lines 40 miles away from the origin won't cause an error IF all the contours themselves and other annotation don't cause the extent of elements to also be larger than the 20 mile threshold. Distance from the origin is one aspect and the total extent (X,Y AND Z) of the elements in the file is the other.

Ultimately, the error appears because they want us to know that this external data could negatively affect the accuracy of what we work with inside Revit.

I wrote THIS POST to discuss how I deal with survey files that violate the threshold. It starts out with one issue (transparent elevations/sections) that occurs when the threshold is crossed.


Dave Baldacchino said...

Revit only cares about one thing: that geometry is within a 10-mile radius SPHERE, with the origin at the fixed internal origin (the Startup Location). You can have the Shared Coordinate Origin anywhere you want, even as far as the moon and more (yes, I did move the SC origin 250,000 miles away for pure geeky pleasure and no error or abnormal behavior resulted from that). I documented this in my AU and RTC class on shared coordinates and as far as I know, all those facts still hold true today. Also thanks to you Steve for helping me understand this topic over the years :)

Steve said...

Dave - In my testing it isn't a sphere, it's a cube. I can put and element outside the sphere but within the boundary of a cube of the same diameter and no error message. Move it outside the cube and error pops. - Cheers!