Author Topic: Surveys to a Autodesk MAP  (Read 3611 times)

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Surveys to a Autodesk MAP
« on: March 09, 2005, 06:59:08 PM »
Curious.. a surveyor asked me today if i could take all his surveys and place them in a gis system using MAP with state plane coord. system...

Is this possible? if so what are the steps?!

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Dent Cermak

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Surveys to a Autodesk MAP
« Reply #1 on: March 09, 2005, 08:43:24 PM »
You can do that in LDD. You do not necessarily need MAP to do this little trick. All of our surveys are now tied to SPCS via our  GPS equipment. Either way you do this, one big, overall data base drawing is going to eat up disk space.
Now, if what you are asking is "Will MAP convert his local project system coordinates to SPC?", then the answer is a big old NO!! He will have to have control base points or GPS coordinates in his project already to do what he wants. To be able  to slide local projedct data into a SPC system is pretty near impossible. Certainly not accurate enough for land surveys. Then there's the issue of getting Project North to True North. Move and twist  automatically ? Can't be done. Not and meet state surevey accuracy standards.
He will have to get GPS coordinates on 2 points on each project. You then can convert the site to State Plane Coordinates and insert each dwg into a master data base drawing. Understand that any survey errors in his projects will be compounded and he will end up with some projects overlapping another at some point. I have yet to fail to see this happen. Common survey points will be found to be not common because of errors that he did not catch before. And I GUARENTEE that there will be errors in easch and every survey and if he did his adjustments (least squares, etc.) different or on different set ups on the same points. it will bite him in the big ole butt!!
And don't get me started about getting aerial photos off the internet and fitting the surveys to state plane controlled photos. If you have a real GIS program that has true "rubber sheeting" capabilities and can make the aerials to orthophoto quality, you might get close. i'd just hate to have to go to court on that kind of data.


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Surveys to a Autodesk MAP
« Reply #2 on: March 10, 2005, 01:14:04 PM »
Everything Dent says is basically true, but you can still manage this.  I know people who do it regularly.  However, it takes some approximation, and this guy has to really ask himself "Why do this?"  He should have a really good reason.  "It seems like a good idea" is not a good reason.

The process involves taking advantage of the fact that each individual "job site" is relatively small, and earth curvature does not matter within the site itself.  Therefore, you can calculate a "conversion factor" for each job.  This is usually a number that is really close to 1, that lets you convert your job coordinates to state plane.  The only way to determine this is to tie down known reference monuments on and near the site in your local coordinate system, and then use that information and the published state plane coordinates for the points to determine your "scale factor" for that particular job.  You then need to scale the whole job by that factor.  You can then place the result in your "state plane" drawing along with other jobs.  Note that EVERY JOB will have a unique scale factor.

Also note that the result is only an approximation.  As Dent says, lines and points from adjacent jobs will not come in exactly on top of each other.  You can end up with a real mess, depending on what you're doing.  So make sure this is REALLY the route you NEED to go down.


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Surveys to a Autodesk MAP
« Reply #3 on: March 10, 2005, 01:21:09 PM »
I should also point out that when you use the process outlined above, you will not necessarily get meaningful results when you attempt to inverse between points located in different jobs.  You are inversing cartesion coordinates, which will not give correct results for widely-spaced coordinates on a curved surface like the Earth.