Plotting a Camera Survey 26

[The camera records pictures that can be taken in camp or on a vacation trip and kept until more leisure may be had in winter for plotting the ground. - Editor.]

A PREVIOUSLY measured base triangle with "stations" at each corner is necessary for making a camera survey, just as it is for the plane-table survey. It is preferable to have each of the three sides measured independently, though if one side has been accurately chained, the other two may be less satisfactorily determined by the use of the plane table. If the camera has a fixed focus, it is possible to make an entire survey from the two ends of a single base line; but this method has no check and should be used only when and where the triangle method is impossible. With an adjustable focus, it will rarely give good results.

Once the triangle has been laid out, the fieldwork is very simple. The camera is set up at one station, carefully leveled, and then a series of pictures is taken, each single plate overlapping the last so as to form a panorama of the area to be mapped. The focus of the lens must not be changed during a series, and plotting is facilitated by keeping the focus constant during all the exposures which make up a survey. To secure good depth of focus, a small stop is generally used, since it is necessary to use a tripod to keep the camera level. If contours are to be drawn, the height of the lens above the ground at the station should be measured and recorded. After a series has been taken at each station, the fieldwork is complete. It is an excellent plan to keep a record of the plate numbers, and the order in which and the station from which the exposures were made, so that the 10 or 13 plates which a small survey will comprise may not get hopelessly mixed up. If the camera is turned each time to the right, clockwise, and the plates are numbered A-l, A-2, B-4, etc., indicating by A-l, for example, the leftmost plate taken at Sta. A; by A-2, the plate just to the right of A-l, just as II is to the right of I on the clock dial, and by B-4, the fourth to the right taken at Sta. B, there ought to be no difficulty in identifying the plates after the exact details of the ground are forgotten.

Plotting a Camera Survey 27Plotting a Camera Survey 28

While the pictures are be ing taken, "flags" of white wood or with white-cloth streamers tied to them must be stuck in the ground or held at the other stations in order that their exact location can be readily and certainly found on

In Plotting a Camera Survey the Base Triangle is First Carefully Laid Out on the Paper to Such a Scale That the Map will be of Desirable Size make the prints from which the plotting is to be done. One of these lines should connect the points at the top and bottom of the plate, and the other, the points at the sides. The vertical line divides the objects which were on the right of the center of the camera from those that were on the left, and the horizontal line connecting the points on the sides separates the objects that were above the camera from those that were below.

If the survey has been made with a lens that does not cover the plate fully or that has considerable uncorrected aberration, causing distorted shapes near the edges and corners of the picture, re-ults can be materially improved by plotting from enlargements. In making the enlargements, the back of the camera should be removed the plates. A few distinctive stakes, some with one and some with two or three strips of cloth tied to them, placed at important points on the ground will help immensely in the location of knolls and shore lines.

Correct Mapping Constant Incorrect Mapping Constant

Ill: Correct Mapping Constant Incorrect Mapping Constant

In plotting a camera survey, either the original plates, the prints, or enlargements may be used. The plates are the most accurate if a corrected lens has been used; and the enlargements made back through the lens will be best if the images on the plates are distorted. In any case, two fine hair lines must be scratched on each plate before it is used to plot from, or to and the light should be allowed to pass through the plate and the lens in the reverse order and direction of that in which it passed when the negative was made. In this way, the errors which were made by the lens originally will be straightened out, and the resulting enlargements will be free from distortion. To make successful enlargements for surveying work, the easel on which the bromide paper is tacked must be square with the camera, and the paper itself should be flat and smooth. It is just as necessary to keep the easel at a constant distance from the camera during the enlarging as it was to keep the same focus while the original negatives were being made.

In plotting a camera survey the base triangle is first carefully laid out on the paper to such a scale that the map will be of a desirable size. With the apex of the triangle representing Sta. A, say, as a center, a circle is drawn with a radius as nearly equal as possible to the distance between the optical center of the lens and the plate when the picture was taken. Ordinarily this will be the focal length of the lens; but if the camera was not focused most sharply on an object a great distance off, the radius may be greater. This radius is called the "mapping constant." When an approximate distance for the mapping constant has been determined by measurements on the camera or by knowing the focal length of the lens, the circle, or rather the arc, FG between the two lines to stations B and C, is drawn. The plates taken at Sta. A, and ranged around this circle on the outside and just touching it, will show the landscape exactly as seen from A.

In the accompanying diagram showing the method of determining the mapping constant and of locating the traces of the plates, the letters F, G, H, J, P, R and S designate points referring to the true mapping constant, and the construction necessary to locate the traces of the plates. The primed letters F', F", G', G", etc., are used to show similar points where the trial mapping constant is either too long or too short. The following description refers equally to the construction necessary with true or trial-mapping constants.

Next, a line FH is drawn perpendicular to the line AB of the triangle at the point F where the arc intersects it. On this line is laid off, in the proper direction, a distance equal to the distance on the plate or print from Sta. B to the center vertical line. From this point is drawn a light line, HJ, toward the center of the arc. Where this line crosses the arc, at J, a tangent, KJM, is drawn, which will show the location of the plate A-l on the drawing. This line is called the trace of the plate. An object which appears both on plate A-l and A-2 is next picked out and its location on the trace of plate A-l determined by measuring the distance JN equal to the distance on the plate from the image of the object to the center vertical line. A light line, NO, joining this last-found point with Sta. A, is then drawn. Where this last line crosses the arc, at O, a tangent, OP, to the arc is drawn, and the trace of the plate A-2 is found with the aid of the point which appears on both plates just as plate A-l was located from the picture of Sta. B. The traces of plates A-3 and A-4 are found in exactly the same way as was that of A-2. If the radius of the arc has been estimated correctly, Sta. C will be found to be exactly on the point where the trace of the plate showing the station crosses the line AC on the paper. If it does not fall on the line AC, which is generally the case, everything must be erased except the original triangle. First, however, a radial line S'G', or S"G", is drawn from the location of Sta. C on the trace of the plate A-2, 3 or 4, as the case may be, to the arc, and the point of intersection of this line and the arc, G' or G", is preserved. If this point, G' or G", is outside the base triangle, the next trial arc should be drawn with a larger mapping constant as a radius, or vice versa. If the second mapping constant is off, find again the point of intersection of the radial line through the new location of Sta. C on the newly located trace of the last plate and the new arc. Join this point and the one found previously, in the same manner, with a straight line, G'G". The point G where this last drawn line intersects the line AC of the base triangle, will be the point through which the arc, with the correct mapping constant as radius, ought to pass, provided the first two approximations were not too far in error. This third trial ought to make the location of the traces of the plates exactly correct. If, however, the focus of the camera was changed between exposures at one station, the traces of the plates will not all be at an equal distance from the station point, and their location will be an almost impossible task. The traces of the plates taken at stations B and C are found in exactly the same manner as were those for Sta. A. After the traces have all been located, it is a good plan to ink them in lightly and erase the pencil construction lines which would otherwise form an impenetrable maze. The traces located, the difficult and tiresome part of the plotting is over ; the landscape, brough indoors photo-graphicall" located as with the plane table; all that remains to be done is to take the sights and find the points on the paper which show where the objects were on the ground.

From Each Station the Mapping Constant is Laid Out by the Focal Distance of the Camera or Distance of the Plate from the Lens, and the Location of Traces of the Plates Determined

Ill: From Each Station the Mapping Constant is Laid Out by the Focal Distance of the Camera or Distance of the Plate from the Lens, and the Location of Traces of the Plates Determined

This taking the sights is a simple matter. With a pair of dividers, the distance from a given object from the center line of the plate is measured. This distance is laid off on the proper side of the point marking the center line of the trace of the same plate; a radial line is drawn through the trace at the given distance from the center-line point and the station at which the given plate is taken; this is one line of sight to the object. The same object is located from another station in the same way; as on the plane table, the intersection of the two lines to the same object marks the location of the point which represents the object on the map.

Obtaining elevations for the drawing of contours is a slightly longer process. Contours are lines joining points of equal elevation; they represent successive shore lines, if the area mapped were inundated and the water , should rise slowly foot by foot. If the contours are close together, the ground represented has a steep slope, and vice versa. If, on a map, a number of points are of known elevation, it is simply a question of judgment and practice to tell where contour lines go.

Before contours can be drawn the elevations of a considerable number of points must be known. If the elevation of any one of them is known and the difference between that one and any other can be found, determining the elevation of the second point is simply a problem in addition or subtraction. If it be desired to find, for instance, the difference in elevation between Sta. C and the corner of the fence, as shown in the sketch, two solutions are possible, as follows:

First: Perpendicular to the line of sight from Sta. C to the fence corner, two lines are drawn, one at the intersection of the trace of the plate by the line of sight, and one at the point on the paper which shows the location of the fence corner. On the first of these two lines is laid off the distance Y', equal to the distance of the ground at the fence post above or below the horizontal center line on the plate. Through this point, on the first perpendicular on the line of sight, is drawn a line through the Sta. C and extended to an intersection with the second drawn perpendicular. The distance from the corner of the fence, on the paper, to this intersection is the distance Y, the difference in elevation from the center of the camera at Sta. C to the ground at the fence post. This solution is longer and less desirable than the second.

Second: In place of perpendicular lines to the line of sight, the trace of the plate, and a line, through the point representing the object, parallel with the trace, may be used.

A datum plane, or reference surface, from which all elevations are measured up to the ground surface must be assumed. The United States Geological Survey uses mean, or average, sea level for the datum in all its topographic sheets. Generally, unless there is a United States Geological Survey "bench mark," a monument of carefully determined elevation referred to sea level, within the limits of the survey, it is better to assume the elevation of some point, as Sta. C, at 100 ft., or greater if necessary to place the datum plane below the ground level at all points within the area to be mapped. Other elevations are figured from the assumed elevation of Sta. C. Allowance must be made for the height of the center of the camera above the ground at Sta. C in computing elevations above Sta. C. All elevations determined for the purpose of drawing contours are ground elevations and not the elevation of the top of objects located on the map. The topographic sheets of the Geological Survey are good examples to follow, in drawing contours. For many purposes, contours are not essential, and the refinements necessary for their drawing may be omitted.