GE 144 Survey and Mapping

Science

Geography Department

University of Dar es Salaam

Dr Danielson Kisanga

Second Semester 2013

Prismatic Compass Survey • Involves the fixing of an object’s position in the field

by measuring the angles of bearings btn the line of Magnetic North (0o) and the line of sight to the object

• The positions of objects several km from the observer can be plotted, thereby allowing larger areas to be surveyed compared to chain and tape survey

• Ground distance measurements aren’t necessary

Prismatic Compass Survey

There are several types of prismatic compass – they are usually along with tape measures, chain, metal markers, notebooks and pencils

To operate the Prismatic Compass

• Open the lid of the compass and flick up the

brass box containing the glass prism

• Align the object with the prism slit and the

hairline on the glass lid

• The aluminum ring or card, marked with the

points of the compass and degrees and

minutes of a circle, is free moving on a

central spindle or floats in alcohol

• A thin bar magnet in the card causes north

point on the card to swing to magnetic north

The prismatic

compass

• Has a free-moving

ring on a central

spindle or floats in

alcohol

• A thin bar magnet in

the card causes the

north point on the

card to swing to

magnetic north

• Hold the magnetic compass steady with

the aid of the thumb ring, then lower your

eye until you see a reading in degrees which

is the bearing of the object

• To show the correct reading the degree

numbers appearing in the prism represent

the degree bearing on the opposite side of

the card

To operate the Prismatic Compass

• Taking reading with the prismatic compass (a) Observer sights an object bearing 78o from magnetic

north (MN)

(b) Observer sights an object bearing 114o from MN

(c) Observer sights an object bearing 287o from MN

Prismatic Compass Surveying

• The degree reading to an object in front of the

observer along a line of sight is the forward

bearing (FB)

• An observer can check a forward bearing’s

accuracy by taking a back bearing (BB) from

the object to his former position

Prismatic Compass Surveying

• Forward Bearing

(FB) to X is 300o, the

Back Bearing from X

to A is 120o

• The Back Bearing

equals the Forward

Bearing minus 180o

• If the FB was 120o

and BB 300o,

subtract 180o from

300o to obtain a

check on the FB

• The rule of thumb is:

– If the FB is over 180o, then subtract 180o to obtain a

BB

– If the FB is less than 180o, then add 180o to obtain

the BB

– All bearings taken during a compass survey should

be checked in this manner

• Intersection is a method of identifying an

object’s position by taking bearings to it from

two or more fixed points and using the data to

fix the object’s position on a map

Prismatic Compass Surveying

Intersection Example • Vodacom phone tower has

recently been erected near the

College

• The college has a 1974 map of

the area and a new Vodacom

tower is to be inserted

accurately on this map

• Two field base stations (A

and B), whose positions are

known on the map, are

selected

• The Vodacom tower , C, can

be clearly seen from both

stations and bearings are

taken to it from A and B giving

a well-conditioned angle at

their crossing point

Resection Example • Resection is the method by

which surveyor’s position on

the map is accurately identified

• Bearings should be taken to

two objects in the field which

can be located on the map

• The readings are converted to

BBs , these bearings converted

to true bearings, and angle

lines drawn from the points to

meet at the observer’s

(surveyor’s) map position

• Two identifiable objects ,

the vodacom tower (Z) and

prominent peak (Y), are

located on the map and in

the landscape

• Forward bearing to Y from

A is 55o00’ and to Z 88o00’

• These are converted to true

bearings as for intersection

by deducting the present

magnetic declination

Resection e.g.

• Back Bearings (BBs) are

computed as illustrated in

intersection

• Fine straight pencil lines

parallel to True North are

drawn through Y and Z,

then converted angles are

marked off clockwise, and

straight angle lines drawn

to meet at the observer’s

position on the map

Resection e.g.

The Open Compass Traverse

• Is designed to provide data in the form of

bearings and linear measurements to fix the

position of route ways and of objects near to the

route way

• Think of a college entrance along an access road

to a main road

• The information is to be entered on a 1972 map

which shows only the winding old road

• Equipment:

– Two accurate prismatic compasses, three or more

ranging poles, tape measures, chains, metal

markers, chalk (for marking tarmac surfaces),

notebooks, pencils

The Open Compass Traverse

Some of the bearings and measurements taken in the field on

an open traverse to fix the alignment of a newly built access

road from college gate to main road for entry onto a 1972 map

Open Compass Traverse Method

• Set up ranging poles at

stations A, B, C, D

• Walk over route and

access length of each

leg by pacing

• Draw a sketch plan

indicating route and

positions of objects

near route

• Measure leg A to B

accurately

• Take FB from A to B, to

the pond and water tank

Open Compass Traverse Method • Measure leg B to C

• Take FBs B to C, B to fish

pond corners

• Check with BBs

• Booking sheet page for

two legs of the college

road open prismatic

compass traverse (AB and

BC)

• The left hand column on

the page shows for each

leg the FB to the next

station, the BB, & mean

bearing (circled) taken as

correct bearing

Open Compass Traverse Method

• At C take FBs along the

path, to corners of

experimental oil palm

plantation and other

objects shown

• Check with BB

• Measure length of road

• Proceed to D

• Observations from D1 are

necessary to fix points

from D

• See the booking sheets

on the right and tabulation

on the next slide

• Tabulation of data for the college road open

prismatic compass survey

Open Compass Traverse Method

• The information is entered on the 1972 map

• Convert all magnetic bearings to true bearings

• If the survey was conducted at the end of 1978

there will be a difference of six years from

publication of the map

• In 1972 the magnetic declination was 6o22’ West

of True North with annual decrease of 2’E for six

year equals to 12’

• E.g. FB A-B equals 53o00’ minus 6o22’ (6o22’ -

6x2’)-46o48’

• Then tabulate the results

Open Compass Traverse Plan

• All distances are

converted to meters to

scale of 1972 map which

1:1000 or 1cm to 10m

• Draw a fine straight

pencil line through A

parallel to True North

and mark off the angle

46o48’ clockwise from

this line with a protractor

Open Compass Traverse Plan

Drawing the Plan

Locating B by bearing and measured

scale distance from A for the final map

Convert the distance A-B of 52.5m

to scale to 52.5mm and measure

this along the drawn line to fix

position B

Fix position of water tower by

intersection from A and B

Draw straight pencil line parallel to

True North through B and repeat

process

The Closed Compass Traverse • Is conducted in the same way as the open traverse

but the survey team returns its original starting point

• Think of the same example

of the college we had an

open prismatic compass

survey

• The team proceeds exactly

as for the open traverse

along each of legs

• Booking sheet for the first

two legs of the closed

prismatic compass survey

of the college grounds

• Check for the

ASSIGNMENT AT THE

END OF THIS LECTURE

The Closed Compass Traverse

• The completed framework

of the closed prismatic

compass survey of the

college grounds

• Scale is 1cm represents

40cm

• This is drawn without

any conversion of

magnetic bearings

because the angles are

correct relative to one

another and the map is

therefore compete in itself

The Closed Compass Traverse

• If the framework and

objects had to be

transferred to an

existing map, would

the bearing have to be

converted?

The Closed Compass Traverse

Avoidance of Errors in Compass Survey

• To reduce possible errors during a compass survey:

1. First check the accuracy of the compass by comparing

with an accurate compass or with location of Magnetic

North for year of survey (deduced from local map with MN

brought up to date and map oriented) – note correction

adjustment for field readings

2. Ensure that compass is held as steady as possible when

bearings are taken; if possible place on a non-metal (wood

or plastic) tripod for most accurate readings

3. Always check every reading with a BB and adjust by

adding or subtracting the mean of the error

4. Ensure accuracy of all chain or tape measurements

between ends of legs and to points of observation on

legs

Avoidance of Errors in Compass Survey

• To reduce possible errors during a compass survey:

5. Check that all conversions from MN to TN are accurate

and all scaled conversions are accurate

6. Ensure booker takes down readings accurately by asking

him to repeat all figures verbally

7. Avoid areas where there are deposits of metaliferous ores,

steel structure, metal gates, railway lines, etc which would

influence readings. Avoid wearing metal rimmed

spectacles, metal bangles or steel watches when using

compass

Advantages of Compass Survey

1. Fairly rapid method in the field

2. A check can be made on all compass bearings

3. Cumulative error is reduced and can be easily

rectified

4. New and distant objects can pin-pointed with

accuracy

5. The method can be combined with others, e.g chain

and plane table, and in fact, the prismatic compass is

nearly always called for in any mapping survey

Disadvantages of Compass Survey

1. In elementary survey, compass

2. READ MORE TO ADD OVER HERE!!!!!!

Take home assignment 1. Compare and contrast chain and tape

surveying versus prismatic compass surveying.

2. Complete booking sheets for the other legs of the closed prismatic compass survey

3. Describe the method for adjusting the closing error.

QUESTIONS???

The End of Lecture Three