To prepare a camera for surveying, it is necessary to arrange that the axial center line through the lens to the plate shall be level, and that the location of the horizontal and vertical center lines shall be indicated on the plate. A spirit level is the best solution of the first problem, and indicated center points of the second.

The spirit level preferably may be of the T-form, with two level tubes, or of the "universal" circular form, with which some hand cameras are equipped. However, ordinary hand-camera levels are generally too rough and difficult of adjustment to insure accurate work. On a view camera, the level may be conveniently located on the bed which carries the lens board. If it is screwed to the under side of the arms it will be convenient for use and out of the way. The bed is likewise a good location for the level on a folding hand camera, while the top of the box is about the only possible location with a box-type instrument.

The cross hairs or center-line indicators should be placed on the back of the camera, just in front of the plate. If indicators are used, fine-thread cross hairs or pencil lines drawn on the ground glass must be used temporarily for making adjustments. Generally, the two cross hairs will divide the plate vertically and horizontally into four equal parts and the hairs or indicators will join the center point of the sides and top and bottom of the opening immediately in front of the plate. But it is essential that the cross hairs have their intersection in a line perpendicular to the plate and passing through the center of the lens. Thus in a camera in which the lens is not placed in the center of the plate, or in which the rising and sliding front has placed the lens off center, either or both of the cross hairs may be off center with regard to the plate.

Camera Preparation 19The Ordinary Round Level may be Used, but It Is Not so Good as the T Level

Ill: The Ordinary Round Level may be Used, but It Is Not so Good as the T-Level

After the cross-hair indicators and the level have been attached to the camera, adjustments are necessary. Surveyors distinguish between permanent and temporary adjustments, permanent adjustments being those for which the instrument maker is responsible, and temporary adjustments being those which can be and are made in the field. The principal permanent or maker's adjustments of the surveying camera are those which insure the center line through the lens, or axial center line, or line of collimation, being perpendicular to the plate, the intersection of the cross hairs being on this line, and that the cross hairs themselves are mutually perpendicular. Temporary or field adjustments must be so made that one tube of the spirit level shall be parallel with the axial center line through the lens and the other parallel with the horizontal cross hair. The first field adjustment is made in the following manner. The camera is set up, complete with thread or pencil-line cross hairs and level, and focused on a stake whose top shall just come to the horizontal cross hair at the center of the plate, when the level tube parallel with the center line of the lens reads level. This stake may be driven to the required elevation or a rod may be held on it and the point where, in the image on the ground glass, it is intersected by the cross hair marked with pencil on the rod as it is held vertically on the stake. The distance to this stake is measured from the camera and another similar stake set at the same elevation by the same method, but in an opposite direction and at the same distance from the camera. The two stakes or the mark on the vertical rod which is held on these stakes in turn will be level with each other, though they may not be level with the camera. The camera is then moved to a point very much closer to one stake than to the other and again leveled. The vertical distance from one stake-top or mark on the rod is measured and the camera then focused on the second stake. If the level is actually in adjustment, the distance from the second stake top or mark will be exactly the same as it was on the first. If not, the difference, or "error," is found between the two vertical distances from the cross hair to the two stake tops. Half this error is corrected by raising or lowering one end of the level tube by means of the threaded nuts which are placed on it for the purpose. The whole process is then repeated until the vertical distances from the horizontal cross hair at the center to the two level stakes, one close to and one distant from the camera, are identical. The axial center line of the lens, or the line of collimation, is then in adjustment with the level. All that remains is to make the horizontal cross hair parallel with the cross level.

The Cross Hairs or Center Line Indicators should be Placed on the Back of the Camera

Ill: The Cross Hairs or Center-Line Indicators should be Placed on the Back of the Camera

This is done by using one marked stake. The camera is leveled as far as the "fore-and-aft" level is concerned and the horizontal cross-hair point at the center marked on the stake. The camera is then swung round until the stake just shows on one edge of the ground glass, the fore-and-aft or longitudinal level being checked to make sure its bubble is still in the center. Then the bubble in the cross or transverse level tube is brought to the center by means of the threaded adjusting nuts, and the camera is thrown hard over so that the stake appears along the opposite edge of the plate. This time, the bubble of the longitudinal level being kept in the center, half the error introduced by turning from one edge to the other is corrected. All of the adjustments are then rechecked, and if they are found correct the instrument is ready for use. If a circular level be used, the method of adjustment is exactly the same, the swing of the bubble along the axis of the camera and transverse to it being used to determine the longitudinal and transverse adjustments. Slips of paper may be used for lifting one side in place of the adjustment nuts of the T-level.

The Maker's Adjustments Should Insure the Line of Collimation being Perpendicular to the Plate

Ill: The Maker's Adjustments Should Insure the Line of Collimation being Perpendicular to the Plate