How to do Quick and Easy Checks on Automatic (Dumpy) Levels.
When you feel that you haven’t the time to do a full two peg test on an Automatic (Dumpy) Level, then these quick and easy checks will give you confidence that you are getting the right results.
Closing a Levelling Run is a Good Check on Automatic (Dumpy) Levels.
Whenever a levelling run is done it should either be closed to a known level point or to the original levelling point. The caveat to this is that if using the original levelling point is that you have at least moved (introduced a change point) the Automatic (Dumpy) Level once, if not more than once during the levelling run. By closing the levelling run you have documented proof of the accuracy of your work and the survey instrument you are using. As a general rule for construction site levels, if your miss-closure on a levelling run is less than 3mm then the levelling run has been adequately done. If you are finding that you cannot close your levelling run to this then carrying out a two peg should be the first check that you carry out. Follow this guide for the correct procedure for carrying out a two peg test.
Checking an Automatic (Dumpy) Level onto a Known Point.
Checking onto a known point early in a levelling run is a very good check. But you need to work it out immediately during your levelling run to know if you can carry on accurately for the rest of the levelling run. This will also give documented proof of the accuracy of your work and of the Automatic (Dumpy) Level you are using but it should also highlight if there is a problem early on. This will save an enormous amount of time and effort, especially if you have a long levelling run to complete. Correct booking of the readings is important, use this guide for the correct procedure for booking using the height of collimation method and this guide for the correct procedure for booking levels using the rise and fall method. The trick to this method is to have unequal sighting lengths, this is in effect doing half of the two peg test, but because the height difference between the points is known, you have in fact done the first half of the two peg test.
Do Half a Two Peg Test to check an Automatic (Dumpy) Level.
This at first may seem an odd thing to do. Surely you can’t do half a two peg test? Well you can, if you know and are confident in the level difference between the two points being checked. If you need to know how to do a two peg test in full then follow this guide for a two peg test. When doing a two peg test the first part of the test is to establish a level difference between two different fixed points that are at least 30m apart. By setting up in the centre of these points, the potential error observed is the same. Then by setting up near to one of the fixed points the potential error observed would be different. This would give level differences between the points that not consistent between the two set ups.
To do a half of a two peg test you would need to know, and have confidence in, the level difference between two known points. By setting up the Automatic (Dumpy) Level near (about 2m away) to your first known point you need to note down the reading to the first known point and then take a reading on the second known point (at least 30m away) and work out the difference in the staff readings. This should be the same as the known difference in the Reduced Levels of the known points.
An ideal way of doing this would be to put in a couple of Hilti nails in a footpath or the site car park, as long as these areas are not subject to heavy vehicles that might cause disturbance to the ground levels and the nails you are using.
Using Steel Columns or Lamp Posts for Automatic (Dumpy) Level checks.
This one may not be quick in the first instance, but will provide a very good way of checking Automatic (Dumpy) Levels at times in the future which can be done just by one person.
First of you need to choose two columns or lamp post that are at about the same level (ground level) as each other and ideally around 50m apart with a good line of sight between them. Choose your two columns or lamp posts (or any suitable vertical surface) and set up your Automatic (Dumpy) Level mid-way between the columns or lamp posts. Remember that these should be at least 30m apart, and ideally 50m or more for more accuracy. Sight the first column and mark it at the level seen on the crosshairs of the Automatic (Dumpy) Level. Take care and mark it accurately. Sight the other column and mark at the level seen on the crosshairs accurately again. If you are by yourself then you could always tape an E-Grad Staff or a tape measure to the column or post to aid the accurate marking on the column or post. Once you have marked a level on both columns or posts then you need to set up your Automatic (Dumpy) Level near one of the marked posts or columns. An ideal way of marking these level points is to use retro targets, these easily stick on to most surfaces. Remember that if your Automatic (Dumpy) Level is accurate these marked levels should be at the same reduced level (Height above Sea Level) and at the height of collimation of the Automatic (Dumpy) Level.
This next step may take some practice to be proficient at but with some practice it can be done quickly and easily. Set the tripod near the one column or post, the top of the tripod needs to be just below the height of the mark on the post. Setup the Automatic (Dumpy) Level on the tripod and level it up. Sight the mark on the column or post and adjust the height of the level until the crosshairs line up with the mark on the post. This may have to be done by adjusting the legs of the tripod first and then using all three foot screws in the same direction for fine tuning to the correct height. At this point the height of collimation of the instrument should be exactly the same as the first setup position of the Automatic (Dumpy) Level. By now sighting the other column or post you should observe that the crosshairs are lining up with the mark on that column or post. If they do not line up then your Automatic (Dumpy) Level is not giving accurate results and needs to be adjusted or calibrated. The advantage of doing the check this way is that it is easier to see the error from the Automatic (Dumpy) Level.
Other Checks and Calibration for Automatic (Dumpy) Levels.
Checking an Automatic (Dumpy) Level should form part of your routine when doing any levelling exercise. Getting your Automatic (Dumpy) Level calibrated every year should also be part of your routine and your local survey supply shop will be able to do this. Getting your Automatic (Dumpy) Level checked every year will give you the confidence that you have carried out your checks correctly but also give other people confidence in your work. You should also remember that the cost of not checking and having your Automatic (Dumpy) Level calibrated could well be having to redo items of work. This could be very costly to do, especially if the materials must be bought again.
Check your Accessories that you are using with the Automatic (Dumpy) Level.
To carry out the above checks you need to ensure that you have all the items necessary and that all of your accessory equipment is in good order. Remember also to check your tripod and your E-Grad Staff to ensure that they are in good order. If they are not, then your results will be poor and you will be blaming the Automatic (Dumpy) Level. So, buy a new aluminium tripod and E-grad Staff.
Top Tip for Getting the Best Results When Checking Levels.
Whenever you are checking an Automatic (Dumpy) Level, or a Rotating Laser Level, or a Total Station, make sure that the tripod head is as close to being level as possible. Whenever I am carrying out these checks I like to use a bullseye spirit level like this one I bought from Amazon. Its 65mm diameter so it spans the tripod access hole and is large enough to be easy to set.
Look after your Automatic (Dumpy) Level.
If you take care of your Automatic (Dumpy) Level then there is no reason why it should not last 10 to 15 years. We have seen numerous Automatic (Dumpy) Levels that are 10 years old that have not had to be adjusted and look like new. To do this remember to always carry the Automatic (Dumpy) Level in the carry box, not on the tripod on site. Setup the tripod first, securing it into the ground and then put the Automatic (Dumpy) Level on the tripod. Finally, never store the Automatic (Dumpy) Level in the box when it is wet, dry it off and leave the box open in a safe location.