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Aim of the Article

The aim of this article is to have some thoughts on the usefulness of applying “corrections” in total stations for use on a “normal” topographical survey.  It is not to instruct the user on how to apply the corrections as this may be found in the manuals for the individual instruments.

What corrections should you consider and when does a correction become significant?

The corrections applied to measurement of distance are :-

1. Prism constant,
2. Atmospheric Temperature and pressure (Met) 
3. Scale factor

The corrections applied to measurement of angles are :-

1. Horizontal Collomation 
2. Vertical Collimation

The final consideration must be the centring and pointing accuracy. The optical Plumb will require checking and with auto lock there may be a targeting correction

When corrections become significant is a matter of argument dependent on the precision being worked to.  Most topographical surveys require a precision to the nearest centimetre or two so any correction that approaches this will be significant.  Typically these surveys will observe detail at a range of less than 100m with control usually no more than 200m apart.

This article will primarily deal with what is usually the most significant of the corrections that the user will input:  – The prism constant error.

Prism Constant (offset)

The prism offset is a correction applied to each distance to allow for the difference between the mounting point centre and the optical centre of the prism.  With Leica Equipment there are a number of different prism constants in use.  The system is designed to have zero constant with the standard LEICA round prism.  Having a stick on target introduces a constant of 34.4mm, the 360 prism has a constant of 23.1mm, or 30mm dependant on model and the mini prism has a constant of 17.5 mm or zero, dependent on model using IR Infar Red measurement.  When used with no prism reflector less i.e.in Red Laser a 34mm constant is applied.  But it is possible to shoot to a prism using RL either deliberately (with the correction applied) or inadvertently with no correction and thus the wrong distance is observed.  This typically happens at the end of a series of non-contact detail when reverting back to the prism. 

With other manufactures such as Topcon and Trimble this is less of a problem with their total stations since they are all set up with a zero constant to a Topcon / Trimble round prism. The prism constant with the Trimble 360 prism is only 2mm.  Direct Reflex (Reflector less) observations require no change in prism constant.

Prisms of different brands

Remember also that if you use a standard Trimble round prism with a Leica instrument you should set a 34mm constant (treat as a tape target in IR).  There are also a number of third party prisms that can be used.

To check and prove that the constant you have applied was correct (and not made matters worse) Tape a short base line (less than 10m) with a steel tape on flat ground and compare the EDM with the taped distance.  Or compare against a previously measured known distance.  Alternatively the 3 peg test may be used.

There is a need to take great care to have the appropriate prism constant set in the instrument.

The 3 Peg Test

For this test three tripods “A,” “B” and “C” should be arranged in a straight line.

The theory is that each part of the length will contain the correct distant plus the prism constant error. 

In order to identify the prism constant correction, the total distance “A-C” is compare to the two parts that make it up (“A-B” and “B-C”)

The whole distance has one prism constant error and the two part distance have two lots of prism constant error.  Thus a difference between the whole distance and the sum of the parts will be equal to the prism constant error which can the be applied    

E.g. if

“A-B“= 20.01m, “B-C“=15.01m then  “A-B“+ “B-C“= 35.02m 

 However if “A-C“= 35.01m the the difference between the sum of the parts and the whole distance is +0.01m.

Therefore applying from above a correction of -0.01m to each distance we have.

“A-B“= 20.00m and “B-C“=15.00m.  “A-B“+ “B-C“= 35.00m 

And “A-C“= 35.00m   

Good practice is to observe the line in the reverse direction and take the mean for calculation, also remember to repeat the test checking the correction has been applied the correct way.

What about other considerations?

The EDM on the total station will have specification good to 2 or 3mm

An error of 50ppm in met correction or scale factor is 5mm per 100m.  (This is most that could be encountered in the UK)

Other considerations such as the centring of the tripod and the pointing may contribute, a further 2mm

Horizontal collimation has no significant effect.

For detail it is the nature of the surveyed feature point and how the detail pole is held that will introduce more error

Vertical collimation and the measurement of target and instrument heights do affect the levels but this will be discussed another time.