7/21/2019 Din 18723Din 18723Din 18723

http://slidepdf.com/reader/full/din-18723din-18723din-18723 1/1

Surveyors have seen the expression“per DIN 18723” associated withstatements of accuracy of theodo-

lites since the introduction of electronictheodolites. DIN stands for  Deutsches In-

 stitut für Normung  which loosely trans-lates into the German Institute for Stan-dards (English language website located athttp://www2.din.de/index.php?lang=en). Atthe time the standard was first quoted by 

manufacturers (mostly from continentalEurope) as an accuracy specification, it

 was actually a draft (~1983). However ithas become widely accepted by manufac-turers worldwide for specifying the accu-racy of theodolites. It should be notedhere that DIN accuracy cannot be inferredfrom the least count of a theodolite, whichis the finest measurement or count that aninstrument is able to make. In fact, withthe advent of electronic instruments, re-liance on the least count for anything butan estimate of precision achievable (notaccuracy) is highly inadvisable.

DIN Spec vs. Accuracy 

The standard is equally applicable tooptical theodolites, and in fact has occa-sionally been used for that purpose. But

 whether the theodolite is optical or elec-tronic, surveyors have tended to assumethat a “5-second” theodolite measures an-gles with an accuracy of 5 seconds. This 

is rarely the case  (except by coinci-dence). The specification is useful forcomparing theodolites however, in thatall theodolites classified as having a DIN

accuracy of 3 seconds, for example, willbe roughly equal in terms of angle meas-urement performance. For purposes of survey design, analyzing data after a sur-

 vey, especially when trying to apply a weight to the accuracy of angle measure-ment, the DIN specification value must 

be interpreted in the light of how the in-strument was used.

If you read the fine print, you are like-ly to find text in manufacturer’s specifi-cations that reads something like “stan-dard deviation of the mean of a face I

DIN 18723 Specification

for Theodolite Accuracy 

HOW THINGS WORK 

Sponsored by Geomatics Industry Association of America

and a face II direction.” Understanding

this is key to using the DIN specification value. If you take a theodolite or totalstation with an angle measuring “accura-cy according to DIN” of 5 seconds, forexample, and observe the horizontal cir-cle reading to a precise target in face I(telescope in the direct position) andthen invert the telescope and take anoth-er horizontal circle reading to the sametarget in face II (reversed position), thespecification tells you that the standarddeviation (that is, confidence level of ~68%) of the mean (or average) of thetwo directions is ±5 seconds. This how-ever is not an angle as surveyors are usedto thinking about. To measure a singleangle, we are required to measure two di-rections (first a backsight, then a fore-sight). Using the equations for propaga-tion of random error, the standard devia-tion of the angle, if measured in face Iand face II and then averaged is 7 sec-onds! (Take the square root of the sum of the squares of the errors in each direc-tion, which in this case simplifies to√ – 2•5.)

Thus the first lesson to be learned

from this discussion is that one needs toapply the rules for random error propa-gation, for the particular way you use

 your instrument. For example, if youmeasure the angle twice in face I andtwice in face II, the standard deviation of the angle will be 5 seconds. If, however,

 you only measure the angles in face I, you can expect the angles to have an un-certainty of 10 seconds.

The second lesson, which may be themore important, is that the method of de-termining the accuracy of theodolites us-

ing DIN 18723 is actually one that meas-ures  precision. As with surveying, by ac-counting as well as possible for system-atic errors, it is possible to arrive at an es-timate for accuracy. Thus if the angles arenot measured in face I and face II, if col-limation has not been checked and ad-justed, if the instrument has not beenleveled properly, if the so-called “heightof standards” adjustment is incorrect, and

so on, then the “fundamental accuracy”of the theodolite as given by the DINspec may not be true at all!

Environmental Influences

In addition to the above-mentionedsources of error resulting from practicesand instrumentation, there are the influ-ences of circumstances, mostly environ-mental (such as atmospheric distur-bances) and the practices and adjustment(or lack thereof) of the accessories that

 will affect the accuracy that is achieved.Examples of these are: optical plum-met(s) adjustment, whether the targetsare prisms or precise traverse targets,

 whether these targets are mounted ontripods or on range poles, or whether thetargets are subject to the problem of “phase” error, particularly if a pole or mi-ni-pole is used as the target.

In summary, DIN accuracy values in-dicated for instruments are not the valuesto be assumed that can be obtained

 when measuring angles. Depending onhow the instrument is used (ignoring ac-cessories and conditions), the angle ac-

curacy may be higher or lower than thespecified value. Once the correct value iscomputed, however, it must then becombined with knowledge of practices,instrumentation and conditions, includ-ing knowledge of the level of adjustmentof all the component parts of the survey-ing system and accuracies.

 Note: Information for this article was 

compiled by the technical staffs of the

GIAA members that manufacture theodo-

lites and total stations.

. . . One needs to apply 

the rules for random

error propagation, for the

particular way you use

 your instrument.

DISPLAYED W ITH PERMISSION • P ROFESSIONAL  S URVEYOR M AGAZINE • November 2002 • WWW .PROFSURV .COM • ALL R IGHTS R ESERVED