Option to export to US Survey Foot for DTM

Currently if you export a GCP tagged ortho image and a DTM they will not line up correctly due to the DTM being exported into international foot. Many people might not even realize this is happening. It seems like a big mistake to have a GCP tagged image and a DTM export not match horizontaly. And it is a huge pain to fix in Autocad.

Eventually the USA will move to the international foot but it is not there yet.

So please give us the option to select international foot or US survey foot. It is critical to have this.

Thank you! :grinning:


Where are you located? We are able to export DTM’s in the correct EPSG no problem.

All you have to do to convert from international feet to survey feet is putting the scale factor of 0.999998 in your CAD project.

Thanks Michael. I’m in the US.

Yes we can convert in autocad as well but I am requesting this a a feature to be added so we don’t have to. The option to have either would be nice.

I guess we still haven’t answered the question of why you are having to download an international feet. As I stated above we are able to download in US Survey Feet on all of our projects and three different EPSGs. I wonder if it’s a plan level issue? Are you using the same EPSG as your other files?

Sorry if it was confusing. I use the same EPSG code for ortho and DTM.

The ortho images come out perfect but the .dxf files (and DEMs I believe - Ill have to double check) we get come out in international feet. Maybe I am missing something.

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What’s your EPSG? It is odd that the same EPSG will download in two different units. The biggest problem we have is when the project itself has a scale factor. What amount of shift are you seeing?

2228 Cal zone 3. The shift is about 15 ft. Would just be nice to have them come in together and not have to worry about someone forgetting to do it. We haven’t noticed any issues when using a scale factor but I’ll keep an eye out. Thanks for your replies. Sorry about late reply. Lots of field work to do.


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I guess I still don’t understand why it is defaulting to international feet… Is there an alternate code? In Texas we have 3 versions of TX-Central that surveyors are using and each have a meters and usft code. One of them also has international feet.

The scale factors may not be an issue in your area if survey and engineering aren’t using them and/or your zones are much smaller. Texas is quite big and you can get pretty far from origin.

I could try code with a different epoch but according to https://spatialreference.org/ref/?search=california it should come out in US survey foot.

California is also a big state but we don’t seem to have a problem with our gcps and orthos when we use scale factors. However we are not doing miles and miles of road. Our big jobs that we do fly with sUAS are subdivision size and at that size often we will opt for an airplane to do the aerial.

I assume you’re using one of the 222x codes? And if you export the ortho and DTM in that same code they come in at different places?

Not that it will make a difference in this situation, but you should probably be using the 64xx codes for NAD83(2011). Spatial is outdated so try going to epsg.io

While Texas and California are approximately the same distance in Y, Texas is 4x the width and we only have 5 zones. Your coordiantes are only in the 6m,2m range and ours are 3m,10m so to us a 1.00001 scale factor is 100ft. Still there’s no way you wouldn’t be affected by a scale factor if your engineers were actually using one. That’s just the nature of a transformation from a Geodetic to a Cartesian coordinate system.

I frequently pull everything into Civil3D and have not experienced this. You are not actually working out of the DXF are you? I usually open the DXF save it as a DWG, then close out of it and open the new DWG and select what I need, hit “Control+Shift+C” for base location I type “0,0” and then import it into the drawing I’m working in with “Control+V” and specify the base as “0,0”. At this point it should line up with the ortho and other survey data for the drawing you are working in.

Is this in response to the USFT vs INTLFT issue or the scale factor effect I was talking about?

Scale factor. If I try to insert the DXF into an existing drawing the scale is always off. Do you experience this also?

It has become more and more common as Surveyors and Engineers are more consistently applying scale factors to their drawings and/or field work. The problem is that rarely does the scale factor exist in the CAD drawing setup. Once you plug it in there most of the problems go away. Regardless I am rarely comfortable in just shifting everything according to the scale factor so I always import the points for our GCP’s into the CAD and then Align the drone data to those GCP’s.

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I just had one where an engineer brought in my progress data to compare with the as-built on a grading project. My data was tied to the GCPs in state plane from the same surveyor that did the as-built. My data was shifted and rotated compared to the as-built. After some investigation, we found out that on the as-built, the surveyor applied a scale factor and a rotation so it would print nice. After they provided a copy of the as-built with that removed, everything lined right up.

The surveyor admitted that in hind sight, they shouldn’t do things like that not knowing what other products and contractors are going to be bringing in layers. I think in the “old days” of a few years ago, they could get away with it because they were the only shop going to be using the data. Nowadays that is not the case and they best leave the data tied to a CRS without “augmentation”.

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Seems all too familiar. Scale factors are fine as long as you know they exist and can find out what they are. Sometimes they are very hard to find on a set of plans in fine print. Rotation on the other hand is a huge No-No. People should have a yardstick taken to their fingers for not knowing how to rotate a model in a paper space layout. One of the first things I do when opening a CAD file from someone else is make sure UCS is set to World and in Carlson there is a nice setting for twist to make sure that due North is up. Something similar might exist in Civil3D as well.

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Agreed. I was really surprised that they would rotate the actual X/Y just to get it to look nice on the print. And then, provide that rotated model to others. :roll_eyes:

Another good tip is that Civil3D and Carlson Civil can natively import Google Earth views. Another up-front thing we do is to bring in the Google Earth image and you know immediately where you drone data is going to come in. One thing I don’t understand is why GCP’s don’t take care of this completely. Sometimes we have to use WGS84 coordinates for our GCP’s because the State Plane XYZ’s are outside the polygon… because of scale factor.

LOL! I work at a survey outfit part time and absolutely hate when other companies do that stuff. If I have to scale pipe inverts off profiles and they do that crap it makes it a mess and 10x more time consuming.