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Open Traverse Adjustment - AutoCAD

Rob Snyder
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Open Traverse Adjustment - AutoCAD

Posted by Rob Snyder on Jan 28, 2011 8:36 am

Hi everyone, thanks for the help earlier.

I am trying to figure out if AutoCAD Civil3D Land Desktop 2008 will adjust an open traverse.  I have a 6 mile traverse and have static GPS control points on both ends and a set in the middle.  We started off from the two GPS control points in the beginning and tied into the ones at the end.

The GPS points were post processed using Trimble Business Center and the traverse adjusted from ground to grid using the combined scale factor.

Any help with if AutoCAD will adjust an open traverse would be appreciated.

Thanks
Rob
Richard Sincovec
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Re: Open Traverse Adjustment - AutoCAD

Posted by Richard Sincovec on Jan 28, 2011 11:09 am


Rob Snyder:

The GPS points were post processed using Trimble Business Center and the traverse adjusted from ground to grid using the combined scale factor.
 
This statement kind of makes me shudder a bit.  But you only went six miles, so you're likely to get an accurate enough traverse, unless you have a lot of vertical change over the six miles.

Sorry, can't help you with how to adjust the traverse in LDD...  I never used LDD for that.
Rob Snyder
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Re: Open Traverse Adjustment - AutoCAD

Posted by Rob Snyder on Jan 28, 2011 3:25 pm

Richard Sincovec:

Rob Snyder:

The GPS points were post processed using Trimble Business Center and the traverse adjusted from ground to grid using the combined scale factor.
 
This statement kind of makes me shudder a bit.  But you only went six miles, so you're likely to get an accurate enough traverse, unless you have a lot of vertical change over the six miles.

Sorry, can't help you with how to adjust the traverse in LDD...  I never used LDD for that.

My I ask why?  And no, not a lot of vertical change.
Loyal Olson Loyal Olson
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Re: Open Traverse Adjustment - AutoCAD

Posted by Loyal Olson on Jan 28, 2011 5:27 pm

 

Howdy Ron,

 

I can't speak for Richard, but I also “shudder” a little whenever I see the term “combined factor” used in a statement that [at least] IMPLIES that a SINGLE factor was used on a traverse of this size.

 

That does NOT mean that you can't do it (in some cases), and get good results (again in some cases).

 

Now whether or not your expectations (and spatial requirements) can be satisfied by a “quick-n-dirty” methodology like this, is NOT something that any of us can really comment on without a lot more information.

 

Proper (and fairly rigorous) traverse reductions are not particularly difficult, and should (IMO) be SOP in this age of computers and data collectors. I certainly UNDERSTAND that software ain't cheap, and we ALL (pretty much) try and get by with what we have on hand to the extent that the results are within our warm-n-fuzzy zone.

 

Back “in the day” I had an HP-41 program that handled ALL of this quite well (NAD27), and many folks were quite surprised how much difference it could make on relatively modest traverses.

 

I guess that I just don't see any reason NOT to run the corrections. Every “leg” (vector) has a somewhat unique “combined factor” (even if it might NOT vary much), and you might be surprised what drops out the other end.

 

Just my 2 bits though...


Loyal
MLB MLB
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Re: Open Traverse Adjustment - AutoCAD

Posted by MLB on Jan 28, 2011 5:48 pm

I don't have LDD 2008. The last version I used was 2006. But there were several adjustment options one of which was the "Open Traverse" aka "Force Closure". Yes, the 2006 version could adjust a traverse between known , held points. But be advised, the GIGO (Garbage In, Garbage Out) rule is in effect. You need to read the Autodesk steps and procedures very carefully. LDD had a Least Squares version, but you had to follow their rules carefully. I found it a bit too limited, so we did all this type of work in Star Net*.
MLB
Paul in PA
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Re: First Question, How Good Is Your GPS ?

Posted by Paul in PA on Jan 28, 2011 6:36 pm

And how was it done?

You have 6 GPS points, are they RTK, post processed only, OPUS-RS, OPUS-S, how many were observed concurrently? etc. etc.

If I had all 6 points within 0.02'H and 0.02'V I would convert grid to ground using 2 separate combined scale factors about the middle pair. The I would set my traverse on one of the end interior points and align it with the opposite pair interior point and see how everything fits. I would the compare all my GPS vector H & V distances againts my traverse H & V distances. Once I know everything is within range I would use an LS adjustment.

I might hold other than my first choices for the adjustment.

I would not consider beginning my adjustment from a close GPS pair to another GPS pair.

Paul in PA

Andrew Gaiennie Andrew Gaiennie
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Re: Open Traverse Adjustment - AutoCAD

Posted by Andrew Gaiennie on Jan 28, 2011 7:20 pm

Why wouldn't you trust a survey with only GPS control? Is it you want to check against a found monument for your personal standards, or have you known of properly done GPS points that were out of tolerance?
Old Pacer
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Re: Open Traverse Adjustment - AutoCAD

Posted by Old Pacer on Jan 29, 2011 11:07 am

I'm not sure, but I think LDD2008 requires the SURVEY add on.  If so, reduce your GPS coords to ground, unless you collected your field in grid.  Load these as 'control points'.  Convert field data or hand key field notes using same point numbers as control points. Adjust and (if your me, fix the mistake and run angain) print out closure for the file.  There are several adjustment programs out there that are easier to use and not very expensive.
Richard Sincovec
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Re: Open Traverse Adjustment - AutoCAD

Posted by Richard Sincovec on Jan 31, 2011 11:36 am


Old Pacer:
reduce your GPS coords to ground

It sounds like you are basically describing the method frequently referred to as "Modified State Plane", where you use a CSF to create a relationship between "grid coordinates" and "ground coordinates".

This is not an ideal approach for a large site.  (I'd actually argue that it's almost never an ideal approach, but that would get off-topic...)  It generally works fine for smaller sites, but we would try to avoid it for any site that extends for more than about a mile.  It can work, and keep you within acceptable error tolerances, but that is dependent on a number of factors.

In some ways, this approach is almost the inverse of the approach that Rob says he used, where he used grid coordinates for the GPS shots, and then used a CSF to come up with an approximate grid distance for each leg of the traverse.  As Loyal said, this approach may or may not cause problems; we'd need to know more about the layout of the area to say anything about that.  But it's not a rigorous approach, such as working in the grid system for all field collection (where each leg of the traverse has its own scale factor) and then doing the traverse adjustment "on the grid".  The rigorous approach is actually not much different, yet it gives you much more reliable results, especially for larger traverses and/or traverses that have a lot of vertical elevation change.
Paul Montero Paul Montero
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Re: Open Traverse Adjustment - AutoCAD

Posted by Paul Montero on Jan 31, 2011 6:00 pm





First I must say that an open traverse by definition starts at a known or adopted position but does not end on a similarly defined position. Such a traverse is also termed an open ended traverse. No closure or precision can be computed.

What you described is a closed traverse. While a closed traverse may end at the same place of the begin point. A closed traverse may also start at a known position and end on a known position (eg.) from a previous survey or a survey with equal or higher precision. Yours is the latter of the two.

In order to analyze your traverse In the Land Desk Top program your field book file should contain the coordinates of the start and end positions at the top. Then the observations in the field book file from start to end will allow the program to make a calculation of the precision or closure. If you use the Least Squares analysis you may constrain more than two positions.

As a side note here in the Massachusetts Mainland Zone the most significant difference between grid and ground distances would result in 4 centimeters for 1000 meters traversed. This means that without scaling your precision would be about 1 in 25000. I would also note grid distances should not be used for physical distances when working on bridge spans or roadways.



See Vayor See Vayor
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Re: Open Traverse Adjustment - AutoCAD

Posted by See Vayor on Feb 1, 2011 1:26 pm

You can adjust a traverse in Civil 3D or LDD (if you have survey). When doing what you discribe I compute my combined factor (not usually on that long of a traverse) and either bring the grid obtained from gps to ground or I used the combined factor in the data collector when I ran the survey in the field. 1. make a fieldbook file from your raw field file. 2. At the top of the fieldbook file add the point no. and coordinates of the control points to the file. Start the survey on one control point backsighting anouther control point. Run the traverse to the ending pair (making sure to end on one control point with an angle to the last one). If needed edit point numbers in the fieldbook file. Then process in Civil D or Survey.

see sample fieldbook file below

Good luck Mike Evans
E-mail if you need help
pls2929@hotmail.com

NEZ 1 10000.00 10000.00 100.00 "PK 1"
NE 124 11959.3834 12360.5839 "PK 63"
NE 110 11931.3809 10769.9622 "PK 57"
NE 100 10021.0652 10626.6618 "PK 48"
NE 28 11966.3330 12755.3421 "PK"
STN 1
AZ 1 9999 268.0429
BS 9999
AD 100 180.0000 627.0158 "PK 48"
STN 100
BS 1
AD 1 0.0000 627.0158 "PK 1"
AD 15 104.3047 346.4473 "IPF 001"
STN 15
BS 100
AD 100 0.0000 346.4473 "PK 48"
AD 17 259.5855 469.8382 "IPF 002"
AD 110 169.5252 1573.6571 "PK 57"
STN 17
BS 15
AD 15 0.0000 469.8382 "IPF 001"
AD 18 268.2148 190.8675 "IRF 003"
AD 110 73.1701 1643.0909 "PK 57"
AD 100 327.1408 630.400624 "PK 48"
STN 18
BS 17
AD 17 0.0000 190.8675 "IPF 002"
AD 19 91.3811 372.9267 "IPF 004"
AD 110 346.2836 1828.0604 "PK 57"
AD 100 255.5710 556.2671 "PK 48"
STN 19
BS 18
AD 18 0.0000 372.9267 "IRF 003"
AD 20 88.1642 293.5225 "IPF 005"
AD 110 64.1508 1958.9388 "PK 57"
AD 100 350.3605 920.8440 "PK 48"
STN 20
BS 19
AD 21 271.4317 679.6128 "IPF 006"
AD 19 0.0000 293.5225 "IRF 004"
STN 21
BS 20
AD 22 90.3459 531.4909 "IRF 007"
AD 20 0.0000 679.6128 "IPF 005"
STN 22
BS 21
AD 24 165.0113 325.6106 "IRF 008"
AD 21 0.0000 531.4909 "IPF 006"
STN 24
BS 22
AD 22 0.0000 325.6106 "IRF 007"
AD 25 232.1930 97.5280 "IRF 009"
AD 110 127.4431 1575.1895 "PK 57"
STN 25
BS 24
AD 24 0.0000 97.5280 "IRF 008"
AD 27 142.3916 591.7830 "IPF 010"
STN 27
BS 25
AD 25 0.0000 591.7830 "IRF 009"
AD 124 233.2715 93.3551 "PK 63"
AD 110 87.4356 1512.8581 "PK 57"
STN 124
BS 27
AD 27 0.0000 93.3552 "IPF 010"
AD 28 212.2240 394.8185 "PK"
AD 110 32.2200 1590.8671 "PK 57"
STN 28
BS 124
AD 124 0.0000 394.8195
MLB MLB
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Re: Open Traverse Adjustment - AutoCAD

Posted by MLB on Feb 1, 2011 1:57 pm

This is not exactly correct.

First I must say that an open traverse by definition starts at a known or adopted position but does not end on a similarly defined position. Such a traverse is also termed an open ended traverse. No closure or precision can be computed.

A traverse that starts one one set of known points and ends on another IS referred to as an "Open Traverse" in many traverse program routines. That is to distinguish it from a traverse that begins and ends on the same point. It is called "open" because the true value of the error contained in the set of measurements made cannot be precisely determined. What is commonly called a "Closed Traverse" is more accurately and completely described as a "Closed Figure Traverse". The word "figure" is almost always omitted. The true and correct value of the error in a closed figure can be more precisely determined.

"Open Traverses" were often called "String Traverses" because they ran courses that represented a "string" between known points. A traverse that neither ends on a known point or its own initial point is described as a "Hanging Traverse". This terminology appears in various Survey texts and software documentation.
MLB
UT Veyor UT Veyor
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Re: Open Traverse Adjustment - AutoCAD

Posted by UT Veyor on Feb 1, 2011 2:30 pm



"It sounds like you are basically describing the method frequently referred to as "Modified State Plane", where you use a CSF to create a relationship between "grid coordinates" and "ground coordinates".

This is not an ideal approach for a large site.  (I'd actually argue that it's almost never an ideal approach, but that would get off-topic...)  It generally works fine for smaller sites, but we would try to avoid it for any site that extends for more than about a mile.  It can work, and keep you within acceptable error tolerances, but that is dependent on a number of factors."

The combined factor should not be mistaken as a "single" factor. Hence the term Combined. Elevation factor & scale factor combined. I'm not a big fan of the term Combination Scale factor (CSF), because it can be somewhat misleading.

As for large areas, using the Combination factor to get ground scaled coordinates is a great approach, especially when making measurements on the ground. We have used ground scaled coordinate systems all the time without any problem. A good approach is to truncate the first couple places of the x and y coordinates just to make sure there is a distinct difference between your grid and ground coordinates.

An example: If an area can be placed within a circle not exceeding 40 miles in diameter (Very Large Scale); and the origin of the system is selected near the center of the circle, the error in the coordinates at the exremities would not exceed 1:60.000 (Very Precise). Where the maximum difference in elevation over an entire area is not greater than 400 feet, no length would be in error by more than 1:50,000. This is just an example, but it gives you a good idea of how large of an area you can use a single ground scaled system.

The simplest approach is to examine the scale factors at the extremeties and the range of elevations and compare these values with those computed for the center of the system and at the mean elevation of the project. If the combined effect on the computed quantities is within acceptable tolerances of the anticipated accuracy of the ground measured data, then the single Project Datum concept is entirely satisfactory. Rarely would a project be of such a size and the topographic relief so severe to require more than one reference system.



Paul Montero Paul Montero
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Re: Open Traverse Adjustment - AutoCAD

Posted by Paul Montero on Feb 1, 2011 4:11 pm





MLB,

I was paraphrasing my "Definitions of Surveying and Associated Terms" for open traverse vs. closed traverse.  However the software I use does refer to and "Open Traverse" as one that starts on a known point and closes on another known point. So I agree with you.

Once upon a time, after getting into a bitter situation with a member of the "profession" who insisted that I had performed an abomination of surveying by using the term open traverse I have since made the distinction found in my post.

Paul
MLB MLB
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Re: Open Traverse Adjustment - AutoCAD

Posted by MLB on Feb 1, 2011 4:31 pm

Okay Paul, so we are in agreement that in the context of Rob's question as it applies to his operation using LDD "Open Traverse" (though sometimes confusing) is not entirely incorrect?

Boisterous arguments over minutia typically waste time and serve little purpose. Complete descriptions along with explanations work better. Tomatoes and "tomah-toes" both make spaghetti sauce.
MLB
Richard Sincovec
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Re: Open Traverse Adjustment - AutoCAD

Posted by Richard Sincovec on Feb 1, 2011 8:45 pm


UT Veyor:

An example: If an area can be placed within a circle not exceeding 40 miles in diameter (Very Large Scale); and the origin of the system is selected near the center of the circle, the error in the coordinates at the exremities would not exceed 1:60.000 (Very Precise). Where the maximum difference in elevation over an entire area is not greater than 400 feet, no length would be in error by more than 1:50,000. This is just an example, but it gives you a good idea of how large of an area you can use a single ground scaled system.




I don't think you're thinking about that correctly...  The error ranges you just specified are those for a Low Distortion Projection over that range.  You would typically not get as good results using a scaled system.

I believe your maximum accuracy would be a function of precisely where you are located within your State Plane Zone (or whatever grid system you happen to be scaling).  I think your statement might be true if your project is centered on the grid zone, since scaling up the grid in that case is doing much the same thing as creating a LDP.  I'm not sure what the maximum error you can expect might be, but it does sound like it would cover a relatively large area - quite a few miles, anyway.

I think most of my dislike for the "Modified State Plane" systems with their "grid" vs. "ground" coordinates is that there's so many ways to mess it up, especially when multiple companies are involved, and not everyone is on the same page.
jwahl
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Re: Open Traverse Adjustment - AutoCAD

Posted by jwahl on Feb 2, 2011 6:06 am

I wonder if Civil 3d is really this limited.  I mean after all these years any surveying software should be able to handle either slope distances or measurements already reduced to horizontal and automatically compute the proper grid and elevation factors for the line and apply them. 

Is the software really that way? In this day of GPS positioning as a primary means of survey control for surveys these kinds of capabilities seem like they are essential no-brainers.

I've used software running in Autocad for about 20 years that does that and also works transparently between geodetic coordinates and projections, so it just astounds me if the native civil 3d routines can't handle any of that, or handle grid to ground conversions in bearing and distance annotations, stationing and so forth.

But then again but I am not going to spend the $$ to upgrade to it just to find out.  Maybe after the next release of Civil 3d I could install the 30 day trial and see what it is really like.  Autodesk told me about 6 years ago that Civil 3d did everything our routines did THEN, but obviously this was not the case. 

The purpose of software should be to allow the software and not the user the capability to compute the conversions necessary to work with native field collected data and drawing elements defined by projections or geodetic values and not require any significant user work arounds. 

If this is not the case you could pre-process your traverse data with another package (which I keep seeing posts indicating that is what people do) to apply the proper scale factors to each traverse leg so that it can be placed properly into the system the drawing is based on.  That still leaves query and annotation functions (that should do the reverse) out of the solution.

Oh well, just sort of venting in amazement about how this is still an issue.

UT Veyor UT Veyor
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Re: Open Traverse Adjustment - AutoCAD

Posted by UT Veyor on Feb 2, 2011 10:45 am


Richard Sincovec:

I think most of my dislike for the "Modified State Plane" systems with their "grid" vs. "ground" coordinates is that there's so many ways to mess it up, especially when multiple companies are involved, and not everyone is on the same page.

The goal for creating the State plane coordinate system was to design a conformal mapping system for each of the states with a MAXIMUM scale distortion of one part in 10,000. Again this is only Scale distortion, and not including elevation factors as well. The "constructive" argument can only be made when you understand the parameters of your specific project, and the accuracies you want to achieve. For me, the projects I have utilized ground scale systems for, have been sufficient enough to meet the accuracies needed for the project. It just depends on the application, but the idea of ground scaling should not be discarded completely/entirely unless you consider the project parameters, and accuracies needed. IT IS WHAT IT IS

Here is a good publication that studies ground scaling in various sites in the US, and worse case scenarios.

I agree, when multiple companies are involved there seems to be more time spent validating the data than it would be to perform the work again with your system. Modified state planes are just one of MANY ways to mess things up when sharing data.
Loyal Olson Loyal Olson
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Re: Open Traverse Adjustment - AutoCAD

Posted by Loyal Olson on Feb 2, 2011 3:40 pm

 

I don't mean to bust ANYBODIES chops here, but I see this conversation as being indicative of the sorry state of “surveying” in general these days.

 

Basically, everybody appears to want to use the latest greatest magic box/gun/robot, but very few really want to understand what the box is doing, or even understand how best to utilize the data that it burps out. Those who point out that there are BETTER (and more rigorous) solutions, are deemed to be JUST “expert measurers” who are caught up in technical minutia.

 

Quick-n-dirty (good enough for “our purposes”) SHORT CUTS and APPROXIMATIONS that made reasonable sense back when we were dragging “chains” (tapes) around, and turning angles with transits and 1 minute T-16s are simply NOT consistent with modern technology and computer capabilities anymore (and really haven't been for DECADES).

 

There is NO REASON (IMO) for surveyors to be prancing around in the field with $50k worth of state of the art equipment (NOT counting the truck), only to take that data back in the office and smear it around as if we were limited to a pencil and piece of paper to COMPUTE with (and using thousands of dollars of [unsatisfactory] software to do it). As several have pointed out above, there IS good software available to do things RIGHT, but little (if any) that will allow one to entirely turn their brain off, and let the HAL-9000 do ALL of the heavy lifting for them.

 

Surveyors as a group, have become OVER-dependent on canned software (often far TOO integrated IMO), that they either don't really understand, or don't care to evaluate deep ENOUGH to realize it's inherent limitations and act accordingly. What's worse, many surveyors are all but required to keep “updating” certain software in order to satisfy clients that have sold their soul (and brain) to the software vendors to whom “surveyors” are little more than a necessary evil, and NOT their prime concern/market anyway.

 

The day may come when artificial [computer] intelligence evolves to the point that a computer can actually “THINK,” but by that time, SKY-NET will be sending R2D2 or T-1000s out there to collect data, and “surveyors” will be extinct anyway.

 

It ALWAYS takes a little longer (always has, always will) to do things RIGHT, but that is ALWAYS (and always has been) FASTER than doing things twice. Or FIXING/explaining something that was “good enough” (at the time), that turned out to NOT BE “good enough” at all.

 

Of course there will always be those who have a gazillion dollars of hardware, software, manpower, and everything else one could possibly need to do everything RIGHT, yet fail to possess any clue as to how to actually use any of it!

 

Rant over...

 

It's Oatmeal Stout Time in the Great American West (even if it is -10F out there).

 

Loyal

Richard Sincovec
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Re: Open Traverse Adjustment - AutoCAD

Posted by Richard Sincovec on Feb 4, 2011 5:32 pm


jwahl:
I wonder if Civil 3d is really this limited.  I mean after all these years any surveying software should be able to handle either slope distances or measurements already reduced to horizontal and automatically compute the proper grid and elevation factors for the line and apply them.
 
I don't actually use C3D for this, but I believe Civil 3D can do this, with caveats...

First, it seems to be based on the "old-style" way of thinking, with a "Sea Level Factor" and "Scale Factor", instead of the newer "Elevation Factor" and "Grid Scale Factor".  The difference, of course, is that the older stuff assumes the grid is at sea level, and it also ignores the geoid.  Ignoring the geoid generally isn't a big deal for this conversion, but assuming the grid is at sea level can be troublesome, especially if you are using a custom coordinate system for a Low Distortion Projection.  I don't think this particularly affects the processing of data, but it definitely affects the way various distances are reported.  There was no provision for labeling both "ground" and "grid" distances at all in C3D until the 2011 version, and in the 2011 version, it is limited to only General Line Labels, making it almost worthless.  And even this level of support is designed with the "Transformation Tab" in mind, which means it is designed to work only with the "Modified State Plane" approach (i.e., using a single CSF for your site to multiply/divide coordinates, and come up with different "grid" and "ground" coordinates for the same point).  The feature returns garbage if you're using a Low Distortion Projection.

Second, when calculating its "Sea Level Factor", it uses a formula different than most other software, in that it only uses the elevation of one of the points.  Most other software will use the average elevation of the two points.  In most cases, the difference is very small and not really significant, but it can raise questions about "Why does C3D fail to return the same results as my other software?"

We find it easier to just do all of these reductions in our data collectors, and use only the coordinate file (e.g. PNEZD file).  This means we are ignoring C3D's Networks, and its ability to do least-square adjustments, but that's generally no big deal.  Other software (such as Star*Net) is much better at that sort of thing.  As far as I know (given that I dont' actually use C3D for this task), C3D's Networks can only apply one set of error parameters when calculating things like Least Squares adjustments, which means it assumes you are using the same equipment for all observations (which is not always the case).  And there is no provision for combining GPS or orthometric level observations into your network, as in Star*Net.  So other software is a lot more capable in this regard.

And finally, C3D seems to handle all of this stuff from a somewhat bizarre point-of-view (it feels a lot like it was created by software engineers who have never actually done any real-world Surveying work, so they didn't fully understand what they were creating). And to top it off, C3D's documentation on this topic is rather pathetic.  So you have to understand the subject pretty well, and then do lots of exploration and testing with C3D itself (or go to a really good training semiar) in order to even get C3D to do any of it at all.

I think that's why so few people seem to be using C3D for this stuff...  Either they don't have a lot of need for adjusting traverses, or they have other software that does the job better, more completely, and more easily.
Bob Webster
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Re: Open Traverse Adjustment - AutoCAD

Posted by Bob Webster on Feb 4, 2011 6:10 pm

Rob

Six miles is a small distance to worry about separate scale factors.  I would join a line in Autocad from the GPS control points at the ends of the survey and corresponding points in your traverse.  I would assume that the GPS is survey grade (not GIS).  Using the allign command, bring the data together.  It does not matter which is aligned to which as this will approximate grid to ground or ground to grid.  The gps set in the middle should confirm the precision of your traverse.  I would not use this technique for alignment of a tunneling machine, but for most applications I think it would suffice.  I would be interested how far the traverse point in the middle differs from the GPS.
Who traverses anymore.  Isn't that what RTK is for?
hristo
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Re: Open Traverse Adjustment - AutoCAD

Posted by hristo on Sep 20, 2011 10:27 pm

How to make the traverse close on the same control point, not at another one?
 
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