The most common and most likely answer to this crossword clue is the 13 letter word TRIANGULATION
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Buddy explains!
The straight part
We know that "What surveyors use" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what surveyors use = triangulation.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TRIANGULATION mean?
TRIANGULATION noun- A technique in which distances and directions are estimated from an accurately measured baseline and the principles of trigonometry; an instance of the use of this technique.
- The network of triangles so obtained, that are the basis of a chart or map.
- A delaying move in which the king moves in a triangular path to force the advance of a pawn.
- A subdivision of a planar object into triangles, and by extension the subdivision of a higher-dimension geometric object into simplices.
- A process by which an unknown location is found using three known distances from known locations.
- The practice of repositioning one's group or oneself on the political spectrum in an attempt to capture the centre.
- (qualitative research) The use of three (or more) researchers to interview the same people or to evaluate the same evidence to reduce the impact of individual bias.