Unit 3 & 4 (Midterms) Flashcards
- a fixed reference line or direction which lies in a vertical plane passing through a fixed point of reference and through the observer’s position
MERIDIANS
- sometimes known as the astronomic or geographic meridian
True Meridian
is the generally adapted reference line in surveying practice. This line passes through the geographic north and south poles of the earth and the observer’s position
True Meridian
- it is a fixed line of reference which line parallel with the magnetic lines of force of the earth. Its direction is defined by freely suspended magnetic needle of a compass held at the observer’s position
Magnetic Meridian
- a fixed line of reference which lies parallel to the central meridian of a system of plane rectangular coordinates.
Grid Meridian
- an arbitrarily chosen fixed line of reference which is taken for convenience. This meridian is usually the direction from a survey station to an adjoining station or some well-defined and permanent point.
Assumed Meridian
- The horizontal angle and direction by which the needle of a compass deflects from the true meridian at any particular locality
Magnetic Declination
- is any deviation of the magnetic needle of a compass from its normal pointing towards magnetic north
Local Attraction
- The angles between adjacent lines in a closed polygon
INTERIOR ANGLES
- The angles between a line and the prolongation of the preceding line
DEFLECTION ANGLES
- the acute horizontal angle between the reference meridian and the line
BEARING
- When the bearing of line is observed in the direction in which the survey progresses
forward bearing
- If the bearing of the same line is observed in an opposite direction
back bearing
- its direction as given by the angle between the meridian and the line measured in a clockwise direction from either the north or south branch of the meridian
AZIMUTH
Types of Compass (7)
- Brunton Compass
- Lensatic Compass
- Surveyor’s Compass
- Plain Pocket Compass
- Prismatic Compass
- Forester’s Compass
- Transit Compass
- one of the most versatile and widely used. It combines the features of a prismatic compass, sighting compass, hand level, and clinometer.
Brunton Compass
- this compass is suitable and sufficiently accurate for preliminary and exploratory surveys, engineering-type surveys, and topographic surveys requiring only low degrees of precision
Brunton Compass
- It consists of an aluminum case containing a magnetic dial balanced on a pivot, a hinged cover with a sighting wire, a hinged eyepiece containing a magnifying lens for reading the dial graduations, and a sighting slot for viewing the distant object
Lensatic Compass
- it is designed for military use such as in reconnoitering, determining directions, orienting maps, artillery fire directions control, and other uses where magnetic azimuths are required
Lensatic Compass
- It has a compass box containing a graduated circle, two sight vanes, a magnetic needle, and two clamping screws
Surveyor’s Compass
- was popularly used earlier for running limited plane surveys of reasonable accuracy. Now it is commonly used for forest surveys, in retracting old land surveys, and in geological exploration
Surveyor’s Compass
- is similar to the surveyor’s compass, except that it has no sight vanes. It is a small hand instrument used for obtaining roughly the bearing of a line
Plain Pocket Compass
- is used in reconnaissance and exploratory surveys
Plain Pocket Compass
- The graduations are found on a rotating card instead of being on the compass box. The card moves with the needle since both are fastened together. Two short sighting devices are provided and magnetic bearing are read by means of a prism at the same instant that the compass is sighted along the line
Prismatic Compass
- it is widely used by sailors for navigation at sea
Prismatic Compass
- Is another type of pocket compass which is usually made of aluminum or some type of metal which does not affect the free movement and positioning of the magnetic needle
Forester’s Compass
- it is widely used in forestry surveys as well as for geological and other similar exploratory surveys
Forester’s Compass
- Is similar to the surveyor’s compass, only it is mounted on the upper plate of the transit.
Transit Compass
- is used to check horizontal angles and directions measured or laid off during transit surveys. It is also used for orienting the transit with the magnetic meridian.
Transit Compass
Sources of Error in Compass Work (5)
- Bent Needle - when a compass with bent needle is used, all observed bearing will have a constant error
- Bent Pivot - the magnitude of the error will depend upon the direction in which the sight is made
- Plane of Sight not vertical – the line of sight may be steeply inclined
- Local Attraction – the correct pointing of the magnetic needle toward magnetic north is usually affected by different forms of local attraction.
- Errors in Reading the Needle - due to the inability of the observer to determine exactly the point on the graduated circle where the needle comes to rest.
Sources of Mistakes in Compass Work (3)
- Reading the wrong end of the magnetic needle.
- Not releasing the needle completely and not allowing it to swing freely about the pivot.
- Mixing or interchanging the recording of azimuths from north and south, magnetic and true bearings, clockwise and counterclockwise angles or forward and back bearings.
Types of Compass Surveys (2)
- Open Compass Traverse
- Close Compass Traverse
o consists of a series of lines of known lengths and magnetic bearings which are continuous but do not return to the starting point or close upon a point of known position
- Open Compass Traverse
o onsists of a series of lines of known lengths and magnetic bearings which forms a closed loop or begin and end at points whose positions have been fixed by other surveys of high precision
- Closed Compass Traverse
- is the projection on a north and south lines. It may be called as north or positive latitude and south or negative latitude.
Latitude of any line
- is the projection on the east and west line.
Departure of a line
West departure is sometimes called
negative departure
East departure is sometimes called
positive departure
- In any closed traverse, there is always an ___. No survey is geometrically perfect until adjustments are made. For a closed traverse, the sum of the north and south latitudes should always be ___.
error; zero
The correction to be applied to the latitude or departure of any course is to the total correction in latitude or departure as the length of the course is to the length of the traverse.
- Compass Rule
The correction to be applied to the latitude or departure of any course is to the total correction in latitude or departure as the latitude or departure of that course is to the arithmetical sum of all the latitudes or departures in the traverse without regards to sign.
- Transit Rule
