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We are currently getting some new survey marks established in the main town of our shire, we have been told that there are different orders based on accuracy: 1st, 2nd and 3rd both for horizontal and vertical. In the main town we already have a few good 1st order horizontal but very few 1st order vertical marks, and we where told that they would just shoot from these marks to make the new ones.

My question is: How do they establish a 1st, 2nd or 3rd order mark in a town that doesn't already have a good mark to use; and how do you know it's right?

I know this might not be directly GIS related but to me surveying is important to GIS.

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3 Answers 3

up vote 8 down vote accepted

In order to establish a mark of a particular order, you have to design the survey so that it meets all the Class and Order requirements (listed in the standards) AND meet the error requirements, which vary depending on the method used (differential or GPS/Trigonometric). The actual position/elevation measurement is fairly trivial, the requirements are designed such that if you meet them, you should meet the closure requirements. Australia has a very well-developed set of national surveying standards, due in part to its history, and land registration system.

Surveyors use existing control points to establish the position/elevation of the new control point. For vertical control, depending on the Class/Order requirements, survey or geodetic-grade GPS or more rarely, differential leveling is used.

High-accuracy GPS surveys involve collections on multiple known points and the new unknown point simultaneously, and the data is then post-processed together to remove systematic error from things like atmospheric effects.

With leveling, you measure the difference in heights between your known points and the unknown point, Level runs start and end on known points, so the error can be quantified very accurately if good techniques are used. This is very labor intensive and requires lots of time and trained personnel, especially if the new point is very far from the known control points.

Generally, the further away the known control points are from new mark, the more work is required for good accuracy, but it is still possible. Systems like OPUS can generate very good results with fairly long baselines and longer collection times.

As for how you can be sure a mark is "right", all (good) survey measurements include a method of quantifying the error and uncertainty. So a mark is considered "right" if its error and uncertainty fall within the accepted standards.

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Thanks for the link to the Australian standards, very helpful. –  Nathan W Dec 2 '10 at 5:48
    
+1 Nice overview. –  whuber Dec 2 '10 at 15:12
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The more points in the 'survey control' the more accurate the data.

Can be denoted as:

First order: f = +/- 0.02(sqrt(M))
Second order: f = +/- .035(sqrt(M))
Third order: f = +/- .05(sqrt(M))

These reflect the closure between established control where f is the maximum misclosure in feet and M is the distance in miles

Standard Survey practice

http://www.dot.ca.gov/hq/row/landsurveys/SurveysManual/Land_Surveys_Interim_Guidelines.pdf

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Survey Marks are the points of known position used to place the conceptual property boundaries we tend to use as cadastre and the real world objects which we can see, like fences. To do this (in Australia, sorry that is where I am) we use a series of Tertiary Network marks which are identified as by the use of triangulation, sun observations, and other Geodetic methods.

Once these marks are set up we then either an identification survey or plan of division is then used to identify where in relation to the legal cadastre the occupation is. Depending on where you are this process may vary.

This may be one where you need to have a word to your local authority to get more info.

Have Fun, CDB

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