Project Base Point Far Away - Any known Issues?

[Skellerman]

I set up a large project (490,00 SF) with a project base point relative to a surveyor's file coordinate system. This placed our project base point and model geometry far away from the project origin.

Are there any known problems with this? I heard it could cause items to appear misaligned. Could it cause lag time in generating views?

[dkoch]

You will find numerous threads and posts discussing this in this Forum, and the consensus is that you want to be as close to the origin point as possible.

You may find this article in Steve Stafford's blog of interest.

[MikeJarosz]

The World Trade Center had its origin located near Easton Pennsylvania, hundreds of miles away. I created a triangle at that origin with an angle of 0°-0'-1" (one second). At the WTC site the side opposite this angle was almost 42 inches! I used it to impress the need for accuracy to the team. You will have problems in Revit with that distance. The books advise no more than a mile or two.

BTW, the WTC started out in Revit in late 2003. I think that was v5.0, but we did that site work in Acad.

[Skellerman]

Thanks for the good info you guys. The article was especially helpful. I will make sure future projects have a nice and close origin point. If my team starts experience visually discrepancies while modeling I'll bring it back to the origin.

Thanks!

[irneb]

I'm not sure I understand why you want to draw at survey angles/coordinates. Your project location (i.e. basepoint / direction) should be associated with your building. You've got shared coordinates to sort out the surveyed stuff.

This would save no end of trouble. Even in ACad it's silly to try and draw at surveyed coordinates in its WCS. I would generally do the same there, i.e.: Draw the building from the WCS's 0,0,0, and make a UCS for the surveyed coordinates origin so I can easily place ordinate dimensions and such by simply swapping to that UCS.

I suppose as Mike's sample with the 100's miles distant origin shows this issue. Generally your survey origin wouldn't be "that distant" though - well hopefully! But that's when using imperial units correct? When I'm using metric units (i.e. mm, around 1/25" per unit) this becomes a huge issue since a survey origin on 5km away already produces noticeable inaccuracies. But such does not happen if I use the Project/Shared idea.

[MikeJarosz]

The WTC is owned by the Port Authority of NY & NJ, usually referred to as the Port Authority (PA). They own the bridges, tunnels, airports and other massive infrastructure of NYC. They are an institution unto themselves. They make the rules, you follow them.

They use a distant benchmark so that all of their property falls in the ++ quadrant for x and y. You would think, with the Atlantic Ocean lapping our shores, that z values would be in single digits, and for property surveyors in NY they are. But not at the PA. Ground level at the WTC is approximately +300 as the PA sees it. That puts their tunnels in the positive z. Everything (except their budget) is in the positive at the PA!

Got that?

[irneb]

Usually we use reduced coordinates here. The closest beacon might be 10's of km away, each beacon would have its own coordinates measured from a central "origin". But then the surveyor would cut off a few digits on the X/Y's (and possibly also the Z - especially since my immediate area is at around 1km above sea level = 3280'). So we end up with survey points never exceeding 10km or so (noted the reduction on the survey drawings).

Now seeing as the surveyor uses m to draw, that means all his coordinates are inside of 10,000m. But the rest of the consultants A/S/MEP all use mm when drawing. So those coordinates become 10,000,000mm units. See how quickly these numbers become huge? Especially since ACad/Revit only uses double precision (i.e around 15-16 digits before floating point errors become unavoidable) - and then you still have to worry about TN orientation which is another hair remover. And to make matters worse, in the metric drawings we don't show fractions - actually we seldom show any dimension not in a factor of 10mm (in Europe we tend to actually draw in 1cm=10mm), so we end up adding some rounding errors into the mix making for individual / overall dimensions which don't tally.

No in all my projects I use a project origin as close as possible to the actual building. Preferably on a grid intersection and with the X/Y parallel to the grids. Coordinate points are listed referring to the Shared Coordinates - which is set according to the surveyor's measurements.

Edit: You might actually find it strange but seeing as this area has lots of mines in it, some of those drawings go into negative Z's (even with our 1km ASL). Now I wonder what such an authority would have done in this case! Go with 0Z = Core of Earth?

[MikeJarosz]

0Z = Core of Earth?

Actually, you touched on an interesting issue. In the US we use a survey standard known as the North American Datum. (NAD) The NAD of 1927 was updated in 1983 using tons of new information acquired from space satellites. This data finally enabled geologists to calculate something they had been searching for since the beginning of time: the center of the earth! Actually, it is the earth's center of gravity. That would certainly be a logical z-origin. The numbers would be kinda big, though. Revit probably couldn't handle it.