Friday, July 14, 2017

20 Mile Threshold on Import

This is a follow up to my earlier post this week regarding the 20 mile threshold. A comment to that post mentioned that the governing extent is equivalent to a 10 mile radius sphere whose origin is at 0,0,0. In my own testing I've observed the threshold is more closely defined as a cube.

This image is a 10 mile radius sphere with a line segment that travels beyond the edges of the sphere but within the boundary that a cube would have.

You can see the highlighted square is the extent of the DWG file and line extends outside of the sphere at each end but is still inside the boundary of where a cube would lie instead.

This image is the same file but the line is altered to extend beyond the edge of the sphere/cube extent.

This is the message that appears when I reloaded the file after altering the line's extent.

The warning can be avoided if we ensure that the DWG file doesn't have any elements that extend beyond the 20 mile cube (10 mile radius). The cube can be quite far from the origin of the DWG file but nothing can be outside the cube's boundary.

Wednesday, July 12, 2017

Reset Shared Coordinates Update

During April 2012 I wrote about using a separate file as a diversionary tactic to allow us to reacquire coordinates from a model we used Acquire Coordinates on before; now that it has changed and no longer lines up with our own work.

In the years since that post Revit seems to have decided it should remember more than one file has had the Acquire Coordinates tool used on it. Revit used to be monogamous but that's no longer true.

The reset process is still necessary but an extra step is required now: we must deliberately disable the link's Shared Site setting first.

Usually it is necessary to move the linked file to align with ours and so its new position can be reacquired. If the setting isn't disabled first it will trigger Revit's desire to change the Shared Coordinate system of the link. Keep in mind that Acquire Coordinates is a pull transaction but moving a file that is sharing coordinates causes Revit to think it must push that change out to the related file. If that's what is really needed then consider using Publish Coordinates instead.

Select the linked file and in the Properties Palette click the Shared Site button (by default says Internal unless someone has changed the name). In the Choose Site dialog that appears click the radio button for Do not share site of selected instance.

It should say <Not Shared> like in the image above after choosing that option. It should be possible to move the linked file into the desired position so it lines up with our model correctly again. If it works correctly you won't get a warning to save the changes to the link nor will you get prompted to do so when you save the file.

It is now possible to link a Reset File to use the Acquire Coordinates on. As soon as that is done successfully the original linked file can be used to Acquire Coordinates again, from it instead.

If the disabling step was not taken we'd find that Revit remembers it has a shared coordinate relationship with both files, the original link and the reset file. Examining properties for both linked files would reveal a Shared Site setting in play (Internal) for both.

However, Shared Coordinates and its Survey Point only acts according to the last file Acquire Coordinates was used on regardless how many files Revit is keeping track of. Trying to use Acquire Coordinates on either file in this condition will just generate this warning.

It's almost as if Revit is treating using Acquire Coordinates like a marriage and keeping a record of each marriage, regardless how many divorces the file goes through. I'd recommend it moves on, focus only on the active marriage and make that work.

To recap - if you find your shared coordinate relationship has failed you'll want a divorce. Then you'll fall for someone else quickly, on a rebound, only to discover that your previous love was the best. Just remember you need to get a lawyer involved to disable your first marriage before you start your rebound. This way you'll legally be able to get married again when you come to your senses.

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.