WHAT IS A TOTAL STATION AND HOW DOES IT WORK?

Posted by Nurul Basher on

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    At Surveytech, we aim to not only provide equipment to our customers, but to support them in their careers with as much information and technical guidance as possible. With that in mind, we’ve decided to produce some educational content to help to clarify what can be a morass of technical language and acronyms.

     

    So first of all, what is a Total Station?

    Well, it’s called a total station because it’s actually the combination of two things:

    • A theodolite - think of it like a telescope mounted on two circular axes, one that goes vertically and the other horizontally. You can then look at a point and measure the angle on your instrument. Theodolites were used to measure out the entirety of Great Britain by triangulation by the Ordnance Survey.
    • An EDM or Electromagnetic Distance measurer - basically just a high grade version of the handheld distos that everyone uses to measure rooms. Not to be confused with EDM meaning Electronic Dance Music - make sure you know your tiesto from your tachometer...

     

    These two things are built into a total station, and so with the aid of a computer, it can measure and triangulate positions, slopes and all kinds of clever things that once would have taken an incredible amount of maths in the field.

     

    And How Does It Work?

     

    Angular Measurement

    When you start up a total station, you’ll see the angular readings in the quick survey page on Leica, or under the TPS page on Geomax. These can be in Gradian/gon (400 in a circle) or Degrees (360 in a circle). Make sure you know what you’re working in before you start! You can change this in Settings.

    When you choose a total station, you have the option of getting one with the angular accuracy of 1”, 2” or 5”. The speech marks denote seconds. One degree is divided into 60 minutes (of arc), and one minute into 60 seconds (of arc). Generally a 5” instrument is sufficient for most tasks, but you might want to consider a 1” one if you’re doing something like monitoring where you need extra accuracy.

     

    Distance Measurement

    Essentially, the total station sends out an infrared signal (a laser) that bounces off the target and it then interprets those reflections to work out the distance covered.

    All very clever, but as a user, you just need to be aware of what kind of target you’ve aimed your EDM at:

    • Is it a wall? In that case, you should choose ‘reflectorless’ or ‘non-prism’. Geomax total stations have up to a 1000m reflectorless range, which is a great feature.
    • A retro? This is shinier than a wall, so set your EDM to ‘tape’
    • A Prism? Prisms are mirrors, so that high reflectivity means that you can measure much further - a mind-boggling 10,000m with Geomax total stations. 

     

    Coordinate Measurement

    This is where the computer aspect of the Total Station comes into its own. The coordinates of an unknown point relative to a known coordinate can be determined using the total station as long as a direct line of sight can be established between the two points. Angles and distances are measured from the total station to points under survey, and the coordinates (X, Y, and Z; or easting, northing, and elevation) of surveyed points relative to the total station position are calculated using trigonometry and triangulation.

    Think of it as a game of battleships, but just with x on one side of the map and y on the other, then y is just used to make the whole thing 3D rather than a flat plan.

    If you have a series of points saved in your total station with known x, y and z values, you can then aim your total station at each of them and work out where it is. This is called doing a Free station or Resection. Make sure that these points won’t move (no good if you need to use them in future!) and that they have good geometry. Remember, it is having to work out where it is using trigonometry, so if they’re all grouped in one place, you’ll get errors. 

    And x,y,z can be replaced by National grid coordinates or whatever cartographic system you want to work in, but the theory is the same. One great feature of the X-Pole system from Geomax is that you can combine a GNSS receiver with a total station so you can do a Free Station using GPS data and start work immediately in National Grid. Really useful if you don’t have any controls already set up…

     

    Applications

    Total stations are mainly used by land surveyors and civil engineers, either to record features as in topographic surveying or to set out features (such as roads, houses or boundaries). They are also used by archaeologists to record excavations and by police, crime scene investigators, private accident Reconstructionists and insurance companies to take measurements of scenes. We also have a few clients who use them to make movie sets - definitely the most glamorous application!

     

    Types of Total Station

    Manual total station, like the Geomax Zoom50 - the user aims manually from behind the instrument, while someone else holds the prism.

    Robotic Total station, like the Geomax Zoom90 - The user can control the Total Station from a distance so works with a tablet connected to the prism pole. Only one person needed. 

    Well, hopefully, that was a useful breakdown all about total stations. If you have any questions or would like to try one for yourself, get in touch!

     


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