Total Stations are accurate, aren’t they?
Total Stations are very accurate pieces of equipment that are very expensive. But what are the greatest errors to affect total station accuracy? When buying or specifying a Total Station we often only concentrate on the accuracy of the Total Station. Most Total Stations on construction sites have an angular accuracy of five seconds or less and a distance accuracy within a couple of millimeters. But even the most accurate of Total Stations can give inaccurate results if the ancillary equipment used with them is in poor condition, not adjusted properly or used poorly.
Do you know what the greatest errors to affect Total Station accuracy are?
In this article I shall be identifying what can severely impact the accuracy that can be achieved when using a total station. Some might surprise you and others may not be a factor for you at all. They are in some order of the significance that they have on the potential accuracy that can be achieved with a Total Station whilst setting out or surveying. With all of what will be said below it is important to remember that the more accurate you want your results to be, the more time you need to spend to reduce the errors present in order to get the most accurate results.
But first of all, let’s remind ourselves of what accuracy means in surveying.
What is accuracy and precision in surveying?
Accuracy refers to how close the measurement is to the true value. In this case when using a Total Station, how close is the Easting, Northing and Elevation to the true value.
Precision refers to how close repeated measurements are to each other. In this case when using a Total Station, how close together are the measured Eastings, Northings and Elevations.
1.) Make sure your tripod is in good condition.
The Tripod that your total station is mounted on is the foundation from which it works. The tripod is most certainly the most overlooked item of equipment that can lead to serious errors from the total station. The tripod should be sturdy enough to support the weight of the total station in use. A quick and easy test to see if your tripod is sturdy enough for supporting the total station can be found here. Remember, never use an aluminium tripod for setting a total station.
Take a look at this video which shows the effect of light pressure being applied to the tripod on the level of the total station. This tripod is in good condition and is capable of supporting the total station in use. Now that we can see the effect that some light pressure on the tripod can have on the total station it should be evident that holding and putting weight through the tripod can have a serious effect on the accuracy from the total station.
The tripod should also be setup on firm, stable ground that will support the tripod and total station throughout the working task. It is worth pointing out that tarmac may well support the instrument in colder weather, but in warm and sunny conditions the tarmac can become soft and the tripod feet can sink into the tarmac. This will not only affect the plumbness of the total station but also the collimation height of the total station.
If your tripod is not in good condition this can seriously affect the accuracy achieved when using a Total Station. Recently, an engineer joined me on a construction site. He was using a Robotic Leica TS16 Total Station and was doing his first resection on the site with it. Using the same site control stations that I have been using (see the guide to doing a resection) he was not able to get an accurate resection. After several attempts at setting up a resection and only getting a 3d quality of 35mm, much greater than the quality required for this type of work. The 3d quality is a measure of the accuracy of the resection, the greater the number the worse your resection is. I would expect to have a 3d quality of less than 10mm for setting out works. The reason for this poor resection setup was the tripod he was using had too much play in the joints near the head of the tripod. Have a look at the video below.
This was a relatively new heavy-duty tripod (about 6 months old) and had no obvious signs of damage but the fixings were loose to the legs and the head. Whenever the total station was rotated the head of the tripod would move and this would alter the plumbness of the total station. In this case, the movement was significant enough for the total station to have to be re-levelled. This tripod is a worry to me, as there is so much play in it. My worry is that this tripod will collapse and fall over with a very expensive total station attached to it. When total stations fall over on site they invariably lead to very large bills, often running into thousands of pounds.
2.) Do Not Just Re-Level the Total Station.
Re-Levelling the Total Station will alter the Height and Position of the instrument.
If the total station does go out of level during the setting out or surveying task then re-levelling the total station will need to be carried out. But before the total station is re-levelled, the reason why it went out of level should be found first. Even if nothing is obvious and the total station is not moved to a new location it would be best practice to complete the entire setup of the total station again.
When the total station is re-levelled, the position that the total station is above your known point also changes. This can be easy to see if the total station is setup over a known point. However, it is impossible to know if this is the case when the total station is setup in an arbitrary position and a resection has been used to set the total station.
If the reason for the total station going out of plumb is not identified, then there is the potential for the total station to fall over.
When the total station does go out of plumb, the following checks would be needed to be done.
- Check the ground conditions that the tripod has been set on. If the ground is soft then there is potential for the tripod to settle into the ground from either the total station turning (if it is a robotic total station) or from the surveyor or engineer is treading around the legs of the tripod. The only solution for this is to move to more solid ground. Also be aware that on hot days that tarmac can become soft and will allow the tripod to settle.
- Check the tripod clamps. Sometimes the clamps for extending the tripod legs may not have been tightened enough to hold the weight of the total station. This is a difficult one to spot, but it is worth just to check the tightness of the clamps. The easiest way to do this is before the total station is setup on the tripod, try to push the tripod down in the direction of each leg. If the leg moves under some pressure, then it is likely that the clamp will need some tightening. Remember that you are only supporting the total station and the pressure put on the tripod should not be excessive.
- Check if the legs are self-supporting. The tripod legs when fully extended should just be self-supporting. Here is a guide to checking your tripod.
- Check for movement in the whole of the tripod. By applying pressure to the head of the tripod, see if you can find play in the tripod head. Look at the video (above) which shows excessive play in a tripod.
3.) The Optical or Laser Plummet being inaccurate on the Total Station.
This is only a factor if setting up over a known point. Follow this quick and easy guide to setting up a total station above a known point. The check for the optical or laser plummet can be done once the setup of the point by observing the known point via either the optical or laser plummet whilst turning the Total Station through 360degrees, ideally at 90degree intervals. If the observed point or laser dot moves significantly then adjustment is required.
What do we mean by a significant amount? For general site setting out or surveying, we would not want to see more than 2mm movement of the point or laser dot.
If you are finding that you are getting significant movement with your laser or optical plummet, then it is best to get it checked out by your local specialist survey equipment supplier. This is most likely something that you can’t alter yourself.
However, this test may highlight that there is a problem with the tripod that the Total Station is mounted on. So, ensuring that the tripod you are using is in good condition is vital.
4.) The Detail Pole Being out of Plumb.
The Total Station only measures to the prism. The point (Bottom of the Detail Pole) of the detail pole is assumed to always be directly below the prism, and thus the position calculated (the point surveyed or set out) is a vertical offset from the prism which is the height of the prism that is entered into the total station. If the detail pole isn’t plumb then there is the potential for the distance measured being either too long, to short or to the left or right. Depending on the way you are holding the detail pole in relation to the total station, then it can be a combination of all of the above.
How to check your Detail Pole.
It is quite easy to check the plumbness of your detail pole in the office. All you need is a good quality spirit level that you have checked is accurate. Using the office doorway (or ceiling if your detail pole is long enough) extend your detail pole so that it is stuck in the vertical position with the bubble in the centre of the circular vial.
Using your quality spirit level, check the detail pole by offering up the spirit level to check the verticality. This needs to be done at 90degree intervals (this is to check both axis’ of movement). If you find that your detail pole is not level, then it will need to be adjusted and this is best done on a special rig at your local survey equipment specialist. If you have the patience, then you can adjust it yourself by plumbing it up and using the adjustment screws under the circular vial.
5.) The Type of Prism being Used and Prism Constants.
The type of prism being used can affect the results given by the total station. Best practice would be to use the same prism manufacturer as the total station manufacturer. This way you can ensure that your prism constants and height of prism stations are correct. This especially applies to aftermarket prisms as often aftermarket prisms will not have a prism constant set, or as I have seen have incorrect prism constants stated. This maybe down to the use of inferior glass or manufacturing tolerances used for the non-genuine prisms and prism mountings.
Why do we have Prism Constants?
There are a couple of main reasons why we have Prism Constants. These are.
- Total Stations use electromagnetic waves (Light) to measure distance. Electromagnetic waves (light) travels through different substances at different speeds. Remember that we need to think of air as being a substance too.
- The physical mounting of the prism will influence its absolute position and therefore distance relative to the base of the detail pole or point of interest must be taken into account.
Both of these points above have to be considered and the prism constant is a consistent offset measurement specific for each type of prism manufactured. This prism constant is applied by the total station for every measurement that is taken.
Make Sure the Prism Constant is Correct.
One of the easiest mistakes that can be made is selecting the wrong prism constant. The above information tells us why prism constants are important. There have been numerous times when I have had the wrong prism constant set in the total station, this is because I switch between a Leica 360 Prism with a 23.1mm constant and a mini prism with a 0mm constant. Most of the time this means I need to do some setting out or part of a survey again.
For a good run down on the Leica prism constants have a look at this website that details them, https://lasersurveyingequipment.com.au/leica-prism-constant-explained/
One final point on prisms to mention is the position of them relative to the detail pole. This does depend on the prism and the attachment type that is used.
On some of the Leica 360-degree prisms there is a yellow arrow that ideally needs to be pointing towards the total station. This is because there is some variance in the prism centres when mounted together. This variance is minor, but it is there. For this reason, if I am using a total station for levelling as well as setting out in plan, I prefer to use a Leica (0mm offset) mini prism.
When using standard circular prisms, it is also worth noting that these should be in good condition and the prism should not droop when being used. This is particularly prevalent on the larger circular prisms. A prism that droops during use will alter the levels being taken and the true offset of the prism. The level issue is particularly prevalent when using robotic total stations that automatically track the centre of the prisms. Have a look at the pictures below to see what I mean.
These are just five of the potential areas (these are the major five) where errors can seriously affect the accuracy of a total station. These are the greatest errors to affect total station accuracy, and these have nothing to do with the total station itself. Any of these problems can wipe out any potential accuracy that the total station has and will lead to inconsistent results. It is best practice to check the condition of your total station accessories at least once a week.