No. 4707 2 DEC 1914

NOTES ON THE USE OF PLANE TABLES AND LARGE

SCALE MAPS WITH ARTILLERY.

1. The following notes are not to be taken as superseding any existing instructions.  They are only intended to explain the methods by which a plane table can be used for ranging in conjunction with a map on a sufficiently large scale.

• If the position of the target can be located from information supplied from an aeroplane, it, and the position of the battery, can be marked on the map and the range and bearing from the magnetic meridian can be measured, and the line of fire given to the guns, or, if the map is mounted on a plane table and then oriented as described below, the line on the map joining the gun with the target will point towards the target, and will be the line of fire.

• On all the Belgian 1/20000 maps the direction of the magnetic north and the angle which it makes with the true north are given, but it is always better to plot the direction of the magnetic north from a meridian when possible.  It must be remembered that the right and left sheet lines of a map are not true north and south, unless these sheet lines are meridians, which is by no means always the case.  It is not the case in the large scale maps of Belgium nor in the Ordnance Survey maps.  The meridians are by definition north and south, and when they are drawn across the map there is no difficulty.  In the case of the 1/20000 maps of Belgium the meridian lines are not drawn across, but in the “B” series the latitude and longitude is given in the margin.  A line joining any two points of the same longitude on the north and south margins will always point true north.

• It is not possible to orient a plane table accurately by the sole use of a trough compass.  The orientation given by a compass must always be considered as an approximation only.  Not only is the local value of the magnetic variation always an uncertain quantity, but each individual trough compass has its own error, and, even if these sources of inaccuracy did not exist, the compass is in itself incapable of any exact setting.
• Draw a north and south line across the map by joining corresponding longitude marks on the upper and lower sheet lines.
• Set off the magnetic variation by a protractor from any convenient point on this line.  The variation is given on the margin of the sheet.
• Set the long edge of the compass box along the magnetic meridian so drawn, and turn the table until the needle points to the centre mark.

The following rules should be observed in setting up a plane table for ranging purposes:-

• Where it is possible to set up the table in prolongation of two well-marked objects, if the sight rule is put on these on the map and the table turned until the sight covers them, the table is then set.
• But if no such opportunity occurs, set up the plane table as carefully as possible by the trough compass.  Then if the table is near any well-marked objects, draw rays to those objects, pace the distances, mark them backwards from the representation of the objects on the map, and the mean position may be taken as correct.  To complete the orientation of the table it is now only necessary to set a sight rule along the line joining the position of the table with any visible distant object marked on the map, and move the table until distant object comes on to the sight.
• It there is no near object to measure from, proceed as follows: – Set up the table by the compass, and resect from three points which should be a mile or two away.  The method is given in Chapter XIV. (edition 1912) of the Manual (see Fig 19 and explanation).  Chapters VII. and IX. Of that Manual should also be carefully read.

• The squares on the Belgian map only fix the position of a point within a square, the sides of which are 500 yards in length, though the descriptions N., S., E., or W., or N.E., S.W., &c., define it more accurately.   If greater accuracy is required the decimal system may be used.  The south-west corner of the smallest squares being taken as origin, and the sides of these squares being considered as divided into 10 equal parts, a point may be accurately defined.

A.

  Thus in this square the point “A” would be 6 3, six parts being measured along the southern and three parts along the western edge.  So far as is known this decimal system has been tried in the field, but has not yet been definitely adopted.

• The following instruments and stores have been issued to all heavy batteries with the Field Army: –
• Plane table.
• Trough compass.
• Sight rule or alidade.
• One sheet of celluloid or gelatine for protecting the map from rain.
• Pencils and indiarubber.
• Indiarubber straps and drawing pins for fixing the map to the plane table.
• Circular transparent protractors.
• A wooden scale graduated to 10,000 yards on the 1/20000 scale on one edge, and 1/40000 scale on the other.

• The following method of locating a target is possible under certain conditions, but the practical value of the system has not yet been proved on service, and it must be considered as being still in the experimental stage:-

Set up two plane tables at two known positions on the map and orient them by one of the methods explained above.  Each observer then fixes a pin in that point on the map which corresponds to his position on the ground, and pivoting the sight rule on the pin, follows the course of the aeroplane.  When vertically over the target the aeroplane gives a pre-arranged signal such as a sudden dip or smoke ball.  At this signal each observer draws a line along his sight rule which will then pass from his position through the target, and the intersection of these two lines fixes the position of the target.  The angles made between these lines and the line joining the positions of the two observers should be measured with a protractor, and communicated at once to each other and to Divisional Headquarters, who should also have a map on which the position of the plane tables in marked.

If the angle made by the two intersecting lines is much less than 30 degrees the error in the position of the target is likely to be considerable.  The best results will be obtained when the rays intersect at right angles.

Greater accuracy can be obtained when theodolites are substituted for plane tables, but the object in each case is the same, viz., to measure the angles made by the lines joining the observers and the target with the line joining the observers.

It is clearly desirable that in each case the observers should be connected by telephone, as it is then possible to make sure that the angles are observed at the same moment, but provided the signal from the aeroplane is unmistakable this does not appear to be absolutely necessary.

It is usual for the aviator to come down and indicate on the map the position of the targets he has observed.  It may happen, however, that although able to see the target he may not be able accurately to locate it on the map, owing to a lack of detail, either on the ground or on the map.  This would often be the case in open spaces such as Salisbury Plain, and, on scales smaller than 2 inches to 1 mile, the detail would often be insufficient to enable the aviator to locate his target on the map.  With good observers, the method just described should fix the position with considerable accuracy on maps on the 1-inch, or even smaller scales.  An observing section is now at the front equipped with plane tables and theodolites, and it is hoped that reports on the success attained will soon be available.

Another small advantage possessed by this method is that the position of the target is known at once without waiting for the return of the aeroplane, provided there is good telephone communication between the observers and Divisional Headquarters.