PLANE TABLE SURVEYING

Surveying Laboratory, Civil Engineering Deptt., CWIT Pune 1.

Pract No. 2Date

Time______ am/pm to _____ am/pm

PlaceCWIT, Pune 1

PLANE TABLE SURVEYING

Object:Study of outfit of plane table & method of radiation & method of Intersection.Instruments:A plane table , A plane alidade , A spirit level , A u fork , A trough compass ,ranging rods 2no.s, A30 meter metallic / plastic tape , chalk pieces etc.

Theory:Plane table surveying is a graphical method of surveying in which field work & plotting is done simultaneously. It is a technique most suitable for filling in of details between the stations previously fixed by triangulation or traversing. The P. T. surveying is not required, as for topographical survey.The plane table consists of a drawing board mounted on a tripod, an alidade & few accessories such as a spirit level, a U fork & a trough compass.Temporary Adjustment of a Plane Table:The adjustments required to be performed at every instrumental set up in order to make the instrument ready for use at that station are called as its temporary adjustments.

Following are the main four temporary adjustments of plane table,

1. Setting Up of Tripod & Fixing the Board Up on It: Set up the tripod such that its legs are spread well apart & top of the table is preferably your elbow level. If the ground is sloping, keep tow legs on falling side & rest one on rising side. Get fixed paper on table.

2. Leveling of The Table: Place the spirit level along the length of the table, on the table & using either ball & socket arrangement at head of tripod or using the leg nearly perpendicular to spirit level, center the bubble. Repeat the process by placing the bubble along breadth of the of table. The process is to be repeated twice or thrice till the table is satisfactory leveled.3. Centering of the Table: Centering is the process of establishing the table at a station such that ground station & its corresponding paper mark are exactly one same vertical line. The table is centered at a station using plumbing fork.4. Orientation: It is the procedure of setting the table correctly with respect to the direction, say magnetic meridian. It is the process of referring the same meridian at a survey station whatever is referred at first plane table station. Orientation is required to be done almost at every plane table station next to first PT station so that lines on table already marked at previous stations are parallel to their corresponding ground lines.Orientation is more important than centering because small angular error in orientation shifts the points through several meters, considerably distorting shape of the traverse or the objects being plotted. Such a distortion is almost unnoticeable on account of error with in permissible limit in centering. There are two methods of orienting the table viz.a. Approximate or Magnetic Needle Method.

b. Exact or Method of Back Sighting.

a. Orientation By Magnetic Needle To orient the table at any subsequent station by this method, the trough is placed along the line representing magnetic meridian, so that magnetic tip of the needle & paper meridian marked at first station is in same direction. The board is then turned, until the ends the needle is opposite the zeros of the scale. The board is then clamped in position. This method is sufficiently reliable, provided there is no local attraction. It is suitable for rough small scale mapping.

b. Orientation By Back Sighting Suppose the table is set up over station B on line ab which has been previously drawn as ab from A. The alidade is placed along line ba. The board is turned until the line of sight bisects ranging rod at A. The board is then clamped in position. Care should be taken to place the table at each of successive station in such a position that centering of the table is least altered, when table is turned for orientation. This can be ensured by keeping the paper lines nearly parallel to their corresponding ground lines. If tables goes out of centre on orientation, but through a distance not more than the permissible, then no need to recentre the table. If scale of plotting is 1:500 & plotting accuracy is mm then one may allow error up to 250 mm in centering. If the table is not oriented, different meridian will be used at each of successive station which is anomalous.

Methods of Plane Table Plane Surveying:The plane table survey of the area can be done by using one or more of the following methods of plane table surveying.

1. Methods of radiation

2. Methods of intersection

3. Methods of traversing.

Method of radiation & intersection are mainly used to locate details from pre-established plane table stations. Method of traversing is basically a method to establish the controls over relatively small area. Method of resection is a technique to establish a plane table station with respect to pre-established control stations in nearby vicinity.1. Method of Radiation:In this method the point is located on plan by drawing a ray from the PT station to the point & plotting to scale along the ray the distance measured from PT station to that Point. This method is suitable for survey of small area which can be commanded from single station. It is mainly used for filling in details from stations which have been previously fixed by triangulation or traversing. Preferably the details near to PT station & where chaining & vision is free are located by method of radiation.

Fig: Method of Radiation2. Method of Intersection:In this method the point to be located is fixed on plan by intersection of the two rays drawn from two independent plane table station from which that point is visible.

Procedure of method of intersection:

Let we want to locate corner no.1&2 of a building by method of intersection. Select two stations A&B from which corner 1&2 are visible position of station A&B shall be such that the triangle form to locate the detail i.e. line 1-2 to locate.

1. Set up plane table at station A & get it leveled see that longer side of the table is parallel to longest line to recorded fix the paper on table such that longer side of the paper is parallel to longer side of the table get marked station A on the paper as a at appropriate location using U fork. Using trough compass get marked North meridian at right hand top corner of paper. Measure distance AB center alidade at a & take radiation to station B & mark point b on radiation at A to some suitable scale. Scale is always selected by comparing on the ground of longest line to be plotted on paper & distance on the paper to accommodate the line. The line abobtained on paper is called as paper base. The scale shall be written at right hand bottom corner of the paper base. the scale shall be written at right hand bottom corner of the paper & preferably above nameplate.

2. Take radiation aa1 & aa2 from P.T station A to corner 1&2to be located.

3. Shift the table to station B set it up there, leveled it & center it in such a way that paper lines are approximately parallel to their corresponding ground lines, orient the table at station B first by approximate method (i.e. magnetic needle method) & then exact method (i.e. by method of back sighting) so that paper lines are exactly parallel to their corresponding ground lines &you are referring same meridian whatever you have referred at preceding os starting station while orientation if centering goes out beyond allowable limit, then again recenter the table & orient it again the procedure is repeated till both centering & orientation are satisfactory. After orientation, clamp the table properly so that it remains stationary & orientation remains undisturbed.

4. Take radiation bb1&bb2 from P.T station B corner 1&2 then point of intersection of radiation aa1&bb1 is corner 1&that aa2&bb2is corner 2.

Fig: Method of Intersection

3. The Method of Traversing:It is a method to establish control station over small estate/property for locating the details there from. It is also used for running survey lines between stations established already. Survey of India has made lot of use of this method to locate the details in topographical surveying. Land records department is also making use of to generate the new records or to verify the old records. Orientation is of ultimate importance in PT traversing; because closing error cannot be corrected here it must be within the specified permissible limit.

Fig: Method of TraversingReference:

For plotting of Plane table survey drawing refer sheet No.___Practical Write ups in Surveying for PTD FE CivilPage 5