Principles of Levelling

Question 1

Levelling

Answer 1

is the operation of measuring vertical distances either directly or indirectly in order to determine differences in elevation.

The difference in elevation between 2 points is the vertical distance between the 2 level surfaces in which the points lie.

Question 2

Geodetic Bench Mark (GBM)

Answer 2

This is a permanent reference point with an established precise elevation. Federal Government precise level crews systematically set permanent reference points throughout each province. Brass or bronze plugs are set in concrete, bridge abutments, building foundation and in solid rock outcrops. Books are printed which list these bench marks and their location and elevation.

Question 3

Bench Mark (BM)

Answer 3

Is a permanent reference point other than the geodetic bench mark. This bench mark has an established precise elevation; however it may or may not be set from the same datum as geodetic surveys. IE: Elevation above mean sea level.

Question 4

Temporary Bench Mark (TBM)

Answer 4

This is a temporary reference point with an established elevation. T.B.M.’s are established for a particular job and given an assumed elevation such as 30m or 300. It could be a spike in a tree, a mark on a hydrant or MH cover, etc.

Question 5

Turning Point (TP)

Answer 5

is an intervening point between 2 BM’s, upon which point BS and FS readings are taken.

Question 6

Backsight (BS)

Answer 6

is a rod reading on a point of known elevation

Question 7

Foresight (FS)

Answer 7

is a rod reading taken on a point of unknown elevation

Question 8

Intermediate Foresight (IFS)

Answer 8

short FS’s for contour grid, etc.

Question 9

Height of Instrument (HI)

Answer 9

is the elevation of the line of sight of the telescope when the instrument is levelled

Question 10

Traverse

Answer 10

a series of connecting lines the length and direction of which is determined. NOTE: A closed traverse is a plat.

Question 11

Elevation

Answer 11

the vertical measured distance above a common datum, usually mean sea level

Question 12

Datum

Answer 12

a common reference or starting point

Question 13

Profile

Answer 13

the line of intersection of a vertical plane and the surface of the ground

Question 14

Contours

Answer 14

lines connecting points of equal ground elevation

Question 15

Ground stakes

Answer 15

stakes upon which the depth of cut or height of fill is marked

Question 16

Objective lens

Answer 16

large lens at the opposite end from the eyepiece

Question 17

Stadia hairs

Answer 17

two parallel hairs within the telescope. A reading, taken on a rod and multiplied by 100 is equal to the distance from the instrument to the rod

Question 18

Station

Answer 18

a point at which a reading is required

Question 19

Tripod

Answer 19

the three legged stand which supports the instrument

Question 20

Cartography

Answer 20

the science or art of map making

Question 21

Interpolation

Answer 21

to find an unknown between two points