# GATE Papers >> Civil >> 2017 >> Question No 18

Question No. 18

The accuracy of an Electronic  Distance Measuring Instrument (EDMI) is specified as $\style{font-family:'Times New Roman'}{\pm(a\;\mathrm{mm}+b\;\mathrm{ppm})}$. Which one of the following statements is correct?

##### Answer : (B) $a$ remains constant and $b$ varies in proportion to the distance being measured.

Solution of Question No 18 of GATE 2017 Civil Paper

Accuracy of EDMI is generally stated in terms of constants instruments error and measuring error proportional to the distance being measured :

$\pm$ (a mm + b ppm)

The first part in this expression indicates a constant instrument error that is independent of the length of line measured.

Second component is distance related error.