ERRORS AND PRECAUTIONS FOR PLANE TABLE SURVEYING

ERRORS AND PRECAUTIONS

  The followings are the common errors in the plane table.

[A]. Instrumental Errors

[1]. The fiducial edge of the alidade might not be straight.

[2]. The vane may not be vertical.

[3]. The surface of the table may not be perfectly levelled.

[4]. The horsehair may be loose and inclined.

[5]. The needle of the tough compass may not be perfectly balanced. Also,

       it may not be able to move freely due to sluggishness of the pivot point.

[6]. The table may be loosely joined with the tripod stand.

The above errors are adjustable or avoidable.

[B]. Personal Errors

[1]. The table may not be centred perfectly.

[2]. The levelling of the table may not be perfect.

[3]. The orientation of the table may not be proper.

[4]. The table might not be perfectly clamped.

[5]. The objects may not be bisected perfectly.

[6]. The alidade may not be correctly centred on the station point.

[7]. The rays might not be drawn accurately.

[8]. The alidade may not be centred on the same side of the station point

       throughout the work.

[C]. Plotting Errors

[1]. A good quality pencil with a very fine pointed end may not have been used.

[2]. An incorrect scale may be used by mistake.

[3]. Errors may result from the failure to observe the correct measurement 
       from the scale.

[4]. Unnecessary hurry at the time of plotting may lead to plotting errors.

The following precautions should be taken when using the plane table.

[1].  Before starting the work, the equipment of the survey work should 
       be verified. Defective accessories should be replaced by perfect 
       equipment. 

[2].  The centring should be perfect.

[3].  The levelling should be proper.

[4].  The orientation should be accurate.

[5].  The alidade should be centred on the same side of the station-pi until 
        the work is completed. 

[6].  While shifting the plane table from one station to another, the tripod 
       stand should be kept vertical to avoid damage to the fixing arrangement.    

[7].  Several accessories must be carried. So, care should be taken to 
       ensure that nothing is missing. 

[8].  The pencil should have a sharp point.

[9].  The distance of the objects or line should be written temporarily along 
        the respective rays until the plotting is completed.

[10]. Only the selected scale should be on the table.

[11]. Measurements should be taken carefully from the scale while plotting.

[12]. The stations on the ground are marked A, B, C, D etc. while station 
        point on the map are marked a, b, c, d etc.

(Next post on “LEVELLING”)