REPORT ON PHOTO MOSAIC ASSEMBLIES.
The following instruction sets out the procedure adopted in compiling mosaics, as currently practised in October) 1951. It supersedes previously issued instructions.
In addition I have supplied a summary on mosaic production from the initial mosaic to the present time, and also some general notes.
Draftsman, (Cartographic Grade II) October, 1951.
NATIONAL MAPPING SECTION
INSTRUCTIONS FOR GUIDANCE OF PHOTOGRAMMETRIC DRAUGHTSMEN
SECTION II PHOTO MOSAIC ASSEMBLY (PMA)
Personnel working on P.M.As. are assumed to be competent in normal slotted template procedure and to have read and understood the extract from the Canadian Surveyor, January, 1950 on azimuth templates (attached).
The P.M.As. normally cover a full map area. Where no ground control exists, at least a measure of cohesion to azimuth and photo scale over the area is effected by azimuth template assembly.
Perimeter runs and one middle run are assembled at mean photo scale. Points established by this assembly are pricked through to the base used,
and alternate photos from the control runs are laid down in their positions on the base. The remainder of the area is covered by photos laid down between the control runs and matched by detail.
Detail, such as rivers, tracks, fences, etc. is marked up with chinagraph pencil, the photos are numbered, and names of any known features are shown. The whole assembly is then photographed in sections. Prints of these sections are assembled to make a composite mosaic. This composite mosaic is photographed to give a negative, at approximately the required scale, from which the final prints are obtained.
Purpose of P.M.As.
(i) Makes available R.A.A.F. air photography of the area, until maps are compiled.
(ii) Field sheets for N.M.S. field personnel.
(iii) Pictorial photo index.
Time of Preparation
Each P.M.A. should be completed in 30 man-days.
1. Outline of Procedure in Assembly of P.M.As.
1.1. Investigation and photo identification of horizontal control available.
1.2. Determination of photo scale.
1.3. Selection of runs for template assembly.
1.4. Measurements computed for grid and control.
1.5. Plotting grid and control on to base.
1.6. Trimming of photo edges.
1.7. Photo work, section of P.Ps. etc., joining runs.
1.8. Making templates.
1.9. Assembly of templates.
1.10. Pricking through fidicual marks.
1.11. Marking up detail on photos using stereoscope.
1.12. Laying of photos of control runs.
1.13. Laying of photo cover of the whole area.
1.14. Marking of sheet corners and photography sections.
1.15. Place photo numbers on the mosaic.
1.16. Mark up the mosaic.
1.17. Write up the mosaic.
1.18. Preparation of title strip.
1.19. Check numbering, place names and general presentation.
1.20. Photography in sections.
1.21. Enter up details in compilation register.
1.22. Strip down mosaic and return photos to library.
1.23. Assemble composite mosaic.
1.24. Stick up title, etc., data.
1.25. Preparation of reduction stick, if necessary.
1.26. Photography of final mosaic.
2. PROCEDURE IN DETAIL
immediate decisions regarding control to be used will be
made by the draughtsman-in-charge from data supplied.
2.2. Determination of Photo Scale
2.2.1. Where a block area is to be covered by mosaics, determine, where possible, photo scale at sufficient points in that block to enable a mean photo scale to be determined.
2.2.2. Lay down alternate photos when obtaining photo distance, since the mosaic is comprised of alternate photos.
2.2.3. Keep a record of photo scale, as computed, for inclusion in compilation register. (see Progress & Checking Sheet)
2.3. Selection of Runs for Template Assembly.
2.3.1. When a block area is to be covered by mosaics, a sketch showing all runs to be used should be placed in the compilation file.
2.3.2. It is current practice to use E.Ks. in preference to W.Ks. where both are available.
2.3.3. Fill in compilation sheet, Sheet A, where runs from adjoining areas have been used for a previous job. This will prevent accidental duplication of work.
For example - along a common edge, runs 1 OR 15 are used, not both. (Note that use of Sheet A is similar to the progress sheet, Section 1, a copy is appended.)
2.3.4. Runs normally used are 1, 8, 15, E.K. and W.K.
This procedure is modified according to the position of control.
2.4. Measurements computed for Grid and Control
2.4.1. A standard form, Sheet B, is used for these computations.
126.96.36.199. A completed Sheet B is appended.
2.4.2. Fill in details on top of sheet. Conversion factor is the factor to convert ground distance to plotting distance on the base material.
Plotting distance (feet) = Ground distance (yds) x 3 / Photo Scale
In this example the conversion factor is 3 / Photo Scale
2.4.3. Enter Latitude and Longitude of sheet corners.
2.4.4. Enter Transverse Mercator projection coordinates of sheet corners from tables.
2.4.5. Adopt a grid that makes the closest fit to the sheet corners, using multiples of 10,000 yards. Enter the Eastings and Northings adopted, on the sketch in the top-left-hand corner of Sheet B.
2.4.6. When a block area is to be covered, use grids which will be common to the adjoining areas.
2.4.7. Compute grid measurements including the diagonal which will be used in plotting the grid 2.5.2. and 2.5.4.
2.4.8. Enter T.M. coords of control points.
2.4.9. Obtain the shortest offsets (yards) from the adopted grid to sheet corners and points of control. This measurement is entered opposite ΔX (difference in yards).
2.4.10. These offsets (yards) are multiplied by the conversion factor to give plotting distances (in feet or inches depending upon the conversion factor used).
2.4.11. Plot in the approximate position of the sheet corners on the sketch in the top left-hand corner of Sheet B.
2.4.12. Sign form B as each section is completed.
188.8.131.52. The checker must not be the computer and is to check all entries as well as computations.
2.5. Plot Grid and Control onto Base
2.5.1. Set out one of the Eastings of the rectangle. Measurements by a steel tape. Use a stretched cord and straight-edge to obtain a straight line in all the following steps. The rectangle is plotted in pencil.
2.5.2. Plot one corner, by describing two arcs, using northing and diagonal distances.
2.5.3. Plot the remaining corner, by describing two arcs, using northing and easting distances.
2.5.4. Compare the two diagonal distances. They must agree to 1/8th inch or the rectangle re-plotted.
2.5.5. Write up the Eastings and Northings on the sides of the rectangle.
2.5.6. Plot in control from the rectangle. Use a steel tape.
2.5.7. All plotting must be checked.
2.5.8. Plotting and checking is signed for on Sheet A.
Note: On (6), (7), (8), (11) - All operations must be signed for in the compilation sheet, Sheet A.
2.6. Photo Work
2.6.1. Selection of P.Ps., Wing Points, Azimuth Points, etc.
184.108.40.206. The procedures as stated in Section I of these notes, particularly "Photo Preparation for Template Production", are to be strictly followed where applicable. It is essential that the quality of photo work does not deteriorate, even though the final results of the work are labelled approximate.
220.127.116.11. Select photos at join points. Reject unnecessary photos. Transfer P.Ps. to joining runs. Complete joins.
18.104.22.168. Show North Points on all photos used for plotting. This assists in titling and assembly of the templates.
22.214.171.124. Select and transfer P.Ps. and Wing Points. Wing Points are to be placed at right angles to the base line and at a distance approximately equal to the length of a base line. Thisprovides the optimum intersection angle.
126.96.36.199. Stereoscopically check above points. P.Ps. MUST BE CHECKED BEFORE PROCEEDING TO MARK A.Ps.
188.8.131.52. Select and transfer A.Ps. Azimuth points are the Points indicating the direction of the P.P. of the alternate photos. Circle these points in green ink, 0.2" diam. Use odd photos as azimuth photos. Complete a whole run including those photos at joins. A.Ps. should be selected as close to the photo edge as is practicable.
184.108.40.206. Stereo check A.Ps., and lines joining alternate P.Ps.
2.6.2. Note on Strength of Angles
Since the control runs form a rough parallelogram shape, the strength of the angles holds the form as flown. Hence particular care should be taken in work on these angles. For this reason, if in doubt whether inclusion of a point will strengthen the join, that point should be included. Care must be taken that obviously redundant points are not included.
2.7. Trimming of Photo Edges
2.7.1. Remove all black edges of the photos, except the fiducial marks and titling.
2.8. Making Templates
220.127.116.11. Prick through positions of points from photos and circle in black ink, 0.2" diam.
18.104.22.168. It is not necessary to mark the scale photo P.Ps. on the azimuth templates, except where they adjoin an intersection of runs.
22.214.171.124. Prick through those fiducial marks which lie along the base lines. These points are used for positioning the photos in the assembly of control runs. Show fiducial marks on azimuth templates only, except where the adjoining area is being compiled at the same time.
Circle the fiducial mark points in black ink 0.2" diam. Mark with red chinagraph cross through the circle.
126.96.36.199. Ring the A.Ps. in pencil as these points are not to be slotted.
188.8.131.52. Score a line with a pricker through the A.P. from the template P.P.
184.108.40.206. To obtain slotting point along this scored line, prick through a point at a distance from the centre P.P., equal to the distance between alternate P.Ps. Estimate this distance. Pencil circle this point (free-hand) along the base line.
2.8.2. Note on slotting distance to azimuth point : the procedure recommended above applies only to a standard run where the distances between P.Ps. are approximately equal. Care should be taken to note if this distance between P.Ps. varies, and the slotting distance adjusted accordingly.
In special cases the slotting distance should be measured from the actual photo concerned. This applies especially at joins.
Estimation of the distance in the standard case is advised because it speeds up the procedure. The length of the slot gives a tolerance of 1" either way, and the exact distance is not required.
2.8.3. Title the template using the stamp provided. The arrow on the stamp is to point North. Place a stamp at each fiducial mark that has been pricked through. Fill in complete information, area, run number, photo number on one stamp. Show only photo number on the other stamp.
Where fiducial marks are not shown on the scale photos, only one stamp need be shown on the template. Refer to 220.127.116.11.
2.8.4. Slot templates.
2.8.5. Trim templates, end only, with print trimmer.
2.9. Assembly of Templates
2.9.1. Normal practice in assembly is followed when good control is available.
2.9.2. When it is essential that azimuth be maintained, for instance in running a template surround in an uncontrolled area lay templates in sequences of six, finding the best fit and fixing the templates in position as the surround is continued. The misclose is measured and a linear adjustment made, treating the angles as constant. This procedure is modified according to the information and control available.
2.9.3. A straight line is drawn the length of the run, on each run, and this line is marked in feet, before the runs are loosened. This is to facilitate re-assembly in the same position.
2.10. Pricking through of Fiducial Marks
2.10.1. Only one fiducial mark for each template is pricked through onto the base, i.e. the one which is uppermost. The templates are pricked through as a complete run before being loosened in any way. When a complete run is pricked through, the templates are pushed to one side and the points circled and numbered on the base.
2.10.2. When the fiducial marks have been pricked through and are being ringed in pencil, an arrow is shown pointing in the direction of the P.P. of the template to which the fiducial mark belongs. This assists in laying the photo control runs, see 2. 12. 2.
2.10.3. At the join of control runs, additional points are pricked through to the base. Where practicable, use P.Ps. for these additional points. These points are used to orient the first photo when laying the control run photos. They are also used to re-establish templates in their correct positions when an adjoining area is being assembled.
2.10.4. Edge strips : where two adjoining areas are to be covered by mosaics, the grid intersection, sheet corners, photo points and several points along the common grid line, are to be pricked through and labelled on an edge strip. This edge strip is then titled and used when a mosaic is made of the adjoining area. It is particularly important that the sheet corners are noted when working along a T.M. Projection zone boundary.
2.11. Marking up Detail on Photos using Stereo prior to assembly of Mosaic.
2.11.1. This step is taken to avoid disruption of the completed mosaic searching for information shown on some maps, but not easily seen without the use of a stereo on the final mosaic.
2.11.2. Mark up ODD photos. Use chinagraph pencil.
2.11.3. As much marking up as is possible should be done by eye.
2.11.4. As the photos are leafed over systematically (see 18.104.22.168) the following procedure is recommended to reduce time in marking up.
22.214.171.124. Select the photos of the run being marked up, also those of the preceding run. Place the photos to be marked up on the right or left hand side of the desk and as the photos are marked up, place them on the opposite side of the desk. At the same time pass over the adjoining photos of the preceding run in a similar manner. In this way, the amount of lateral overlap can be seen for every photo. Allow about an inch for North-South duplication. Sketch a rough E.W. line, an inch N. of the line of overlap on the photo being marked up. Mark up to this line.
126.96.36.199. Treat the E.W. overlap similarly.
188.8.131.52. Since this procedure is designed to reduce time in marking up country with heavy detail, no time should be spent in meticulous matching of prints. Where the overlap is consistent, only every second or third photo need be compared.
184.108.40.206. This method will also aid in maintaining continuity of tracks, etc., as a picture of the country can be followed down run by run. If a feature, track, etc. is believed to continue to another run, it should be searched for at this step, under correct stereo. conditions instead of on the mosaic.
2.11.5. Cultural features, i.e. tracks, railways, homesteads, etc. are the features to be shown on this preliminary marking up. Marking up of streams, etc. is to be left for marking up on the mosaic. In many cases, tracks follow a stream and marking up the stream will obscure the track, which consequently is not shown.
2.11.6. Work in close consultation with existing maps and search for features shown on those maps so that there is no need to make such a search upon the mosaic.
2.11.7. Preliminary marking up must be firm as there is a tendency for it to be erased when working over the mosaic.
2.11.8. As each run is marked up, sign for it in the compilation book, sheet A.
2.12. Laying of Photos of Control Runs
2.12.1. Lay down first photo to maintain orientation of the run.(See 2.10.2.)
2.12.2. With subsequent photos, fix a pricker into the table through the fiducial mark on the photo and on the paper base. Swing into line by matching detail with the preceding photo. Where runs are distorted in any way, line up the photos with a straight edge, using the two P.Ps. and the A.P.
2.12.3. To assist in identifying which fiducial mark to use, and which end of the run to commence laying, an arrow is marked on the base. This shows the direction from the fiducial mark, of the P.P. of that photo. Orient the photo and it can readily be seen which fiducial mark has been used and therefore which end of the run to commence laying the photos.
2.12.4. E.W. runs are to be fixed to the base along their northern edges. Key runs are overlaid by the E.W. runs and can be fully fixed down.
2.12.5. When control photos have been laid down, check for any mistakes in the laying or original pricking through.
2.13. Laying of Photo Cover of the whole Area
2.13.1. Odd numbered photos are used for filling in the mosaic.
2.13.2. E.W. runs overlay key runs.
2.13.3. Runs are "leafed over" in a systematic manner, Run 1 overlays Run 2 which overlays Run 3, and so on.
Consistent black corners to photos are trimmed off where they are showing.
The overlay sequence is only changed where tone matching of the runs would be greatly improved by such a change.
2.13.4. Where an "A" run is used to cover a gap less than an inch in width, place the "A" run underneath the runs between which the gap occurs.
2.13.5. Full photo coverage of the area must be given if the photos are available. This is particularly important for comparing sheet corners where adjoining areas are compiled at a later date. Photos from adjoining areas may have to be used; where possible have a 3" border of photo coverage outside the adopted sheet corners, because sheet corner positions are only approximate.
2.13.6. Filling in the mosaic:
(a) The normal area is divided into two parts by the central run, usually No. 8.
(b) This is again subdivided by using the centre key run.
(c) Each quarter section is now further divided into nine equal areas using two E.W. runs and matching photos along two N.S. lines. The runs used in this final division are adjusted to average out any error or misfit found in the quarter section.
(d) These runs are then fixed down with tape along the northern edges.
(e) It is important that the character of each run be maintained (i.e. crabbing, etc.) and the orientation of the photos must be watched carefully as well as averaging out any scale change.
(f) The area has now been averaged out for error and misfit. The whole area is now filled into the runs laid down as in (a)-(e) and it is possible to attempt some continuity of features (i.e. railways, rivers, etc.) without having a cumulative, gross error in any section of the sheet.
(g) This procedure is modified to conform to any existing control.
2.13.7. When assembling photos on mosaic, chinagraph each photo with its number and place the full photo number and run number at least three times per run. This is to facilitate inking in of the numbers.
2.13.8. Check that no significant marking up is obscured before finally taping down. This should not be necessary if procedure in 2.11. is observed.
The following procedure is adopted to obtain matching of
detail along a common edge between two mosaics.
This only applies when the control run does NOT run along the sheet edge.
(a) The edge with full photo cover is marked up as in 2.14.
(b) On the area without full photo cover of the edge, that sheet edge is drawn in pencil on the base and the intermediate crosses shown.
(c) Where the adjoining area is already completed and photographed, the same photos are used and placed in position using the line for orientation and the crosses to average out for scale.
(d) Where both areas are being prepared at once, the position of crosses and pencil line is transferred to those alternate photos not already in use, and those photos laid down as in (c).
For East and West edges, the sheet edge is divided into feet to facilitate transfer of detail. This is not normally necessary for North and South edges.
(e) Where the original photos are not available, the boundary positions should be obtained from the mosaic to which the comparison is to be made.
2.14. Marking of Sheet Corners and Photography Sections
2.14.1. Note: The following steps are to be taken consecutively. They are to be completed in the one operation.
2.14.2. Check against adjoining areas to see if the sheet corners are shown on an existing mosaic. If so, transfer these points onto the photos used in the present assembly. Preferably use the original photos, otherwise the final mosaic.
2.14.3. This transfer must be checked. Refer to Progress and Checking Sheet.
2.14.4. Place the point as fixed in step 1, over the plotted position on the base, or where there is no adjoining mosaic, transfer the sheet corners from the base to the overlaying photo.
2.14.5. Check step 4. Refer to Progress and Checking Sheet.
2.14.6. Join the sheet corners by a pencil line using a stretched cord and straight edge. Use a soft pencil so as not to cut the photos.
2.14.7. Divide the distances between these points, N.S. into half and E.W. into thirds.
2.14.8. Join these points with pencil lines. This means the sheet is cut into six approximately equal rectangles which will be used in photographing the mosaic assembly. They can be used when necessary for rectification.
2.14.9. Place ink crosses at the intersection of the pencil lines. Sheet corners 2" crosses (each arm 1")
Intermediate - crosses, 1" crosses (each arm ½")
Use No. 3 Uno Pen.
2.15. Place Photo Numbers on Mosaic.
2.15.1. Use U.F. 8 Uno Guide with No. 5 pen.
2.15.2. Form of numbering, e.g. 9/5006.
Run numbers and full photo numbers are shown three times for each complete run, at each end of the run and in the middle of the sheet.
2.15.3. Where practicable, numbers should keep to an even line. (Not necessarily straight).
2.15.4, For each run the run number and full photo number should be placed within the sheet edges. The area outside a 3" border from the sheet edges does not appear on the final mosaic.
All photos within that 3" border must be numbered. Owing to the fact that limited control has been used in fixing the edges of these mosaics, it is possible that when further control becomes available, the position of the sheet edges will be altered To avoid the possibility of un-numbered photos appearing on the revised mosaic, numbers on photos near the sheet edges should be placed in that part of the photo closest to the sheet centre, so that if any part of the photo is shown on the final mosaic, then it can be identified.
2.15.5. Purpose of the numbers is to identify the photos and they must be clearly visible. The numbering must not conflict with the marking up or writing up. It should be placed on a suitable background, even if this breaks the continuity of line with the remainder of the run.
2.15.6. Where photos from an area to the East or West are used to complete photo coverage, only the last two numbers of the photo number are shown on those photos.
2.15.7. Numbers must not cross the pencil lines joining the sheet corners. They can cross the internal pencil lines.
2.15.8. With the exception of "A" or "gap" runs underlying other runs (where the gap is about 1" in width), all photos shown must be numbered.
2.15.9. In those cases where white ink is used to show photo numbers on the black background of the sea, use a No. 3 pen with the guide. The finer pen work will prevent the numbers conflicting with island names, etc.
2.16.1. Time Factor in Comlilation of Mosaics.
220.127.116.11. These notes apply tc steps 2.16.2. and 2.17.
18.104.22.168. Distances specified on the conventional signs, spacing in the writing up and line thickness in marking up are to act as a guide and reference.
22.214.171.124. THEY DO NOT HAVE TO BE INDIVIDUALLY MEASURED FOR EACH JOB.
126.96.36.199. After some experience, a regular reference should serve to maintain a close approximation to the standard required. This will ensure a uniform appearance of marking up and writing up over the whole series of P.M.A's irrespective of change in personnel.
2.16.2. Marking up the Mosaic
188.8.131.52. Regular, firm, black lines, using a chinagraph.
184.108.40.206. Relief. Where hills are isolated, a line should be run round the base. This is not necessary where there is a large area of mountainous country. See mosaics of Prince Regent and Lake Amadeus for a comparison.
220.127.116.11. The form of the creeks should be retained, and obscured as little as possible, while keeping a firm black line. This makes the mosaic more useful to the field men in identifying themselves on the ground.
18.104.22.168. The general idea is to emphasize detail which is likely to be lost or hard to identify at the final reduced scale.
22.214.171.124. Refer to the Mosaic Standard, for conventional signs and general standards to be maintained in presentation.
2.17. Writing up the Mosaic.
2.17.1. The authority used in the case of spelling where there is a conflict between different maps of the area, is the map published under authority of the appropriate Surveyor General.
2.17.2. THE MOSAIC IS A PICTURE OF THE AREA. It should not be obscured by a greet number of names and descriptive data. Sufficient should be shown so as to make for easy reference and identification on the mosaic.
2.17.3. All lettering to be done using U.N.O. Use ½" capitals, but otherwise comply with current specifications as to upright or italic.
2.17.4. The small prints after initial photography are joined into a composite sheet along photo edges. Therefore in proximity to a join, no descriptions should overlie photo edges. In any case, descriptions overlaying photo edges should be avoided.
2.17. 5. Notes on Writing up the Mosaic.
These notes must be followed, where practicable. Refer to examples in the Mosaic Standard for spacing, etc.
126.96.36.199. In general, lettering should be written across the map from left to right,in lines parallel to the top and bottom sheet edges.
188.8.131.52. Writing associated with a "running" feature (river, range, etc.) should be written along the feature and on one side of it, with a left to right tendency. The writing should be aligned along simple curves rather than in close parallelism with the irregularities of the feature.
Place on northern side of the feature where practicable.
184.108.40.206. Multiple work names, including those of "running" features and notations, should be compact and in a straight line or along simple curves.
THE DESCRIPTION SHOULD IDENTIFY ITS FEATURE IMMEDIATELY.
If it is too open in assembly it may be lost among the details of the mosaic picture.
220.127.116.11. All names should be so placed that they will be readable from the southern or eastern edges. Names that are lettered vertically should read from south to north.
18.104.22.168. All town and feature names should be on the right hand
side of the town or feature, where convenient.
22.214.171.124.Names anchored to "fixed" features should receive priority over those less rigidly localized.
2.18. Preparation of Title Strip.
2.18.1. The title strip is for purpose of identifying the negatives.
2.18.2. Information shown should include a scale strip, date of photography of the original mosaic, sheet name and number and position for the sub-section letter. See sample in Mosaic Standard.
2.18.3. Lettering and Numbering. UC 8 and UF 8 Uno guide. No. 5 pen.
2.19. Checking Procedure for Mosaics.
2.19.1. Three checking sheets are used A, B and C and these sheets cover all essential checking operations for steps 2-18.
2.19.2. Sheet A covers steps 5, 6, 7, 8, 11; sheet B covers step 4 and sheet C covers the remainder. Each operation must be signed for upon completion.
2.19.3. When a mosaic is ready for photography all items on the three sheets should have been initialled.
2.19.4. In addition to checking that the above items have been completed, the officer who certifies the completion of the mosaic, is responsible for the overall appearance and standard of the mosaic.
2.20. Photography in sections.
2.21. Enter up details in compilation register.
2.21.1. A loose-leaf compilation file is kept, which covers all areas of which mosaics are being compiled. Each area has Sheet A, B and C and a compilation register sheet.
2.21.2. After initial photography (2.20) production details are entered in the register sheet. Date, place, camera used, number of negatives, type of negative and approximate scale of the negative.
2.21.3. At this stage entries under compilation data, can be completed and signed by the Officer-in-Charge. The entries are, in the main, an extract of information on sheets, A, B, and C which can be now removed from the file. The register sheet is placed in its file. This means that only areas being compiled are kept in the compilation file.
The process to be followed in obtaining the final mosaic from photography in sections, 2.20, will vary according to the equipment available and the type of final mosaic required.
EXTRACT FROM THE CANADIAN SURVEYOR.
VOLUME X. NUMBER 3. JANUARY, 1950
A SUGGESTED IMPROVEMENT IN THE SLOTTED TEMPLATE METHOD OF CONTROLLING
VERTICAL AIR PHOTOGRAPHY
By S.G. GAMBLE
Topographical Survey, Mines and Resources, Ottawa.
The slotted template method has been thought to give a better answer in positioning vertical air photographs than graphical methods. It has also been considered as a means of reducing the ground control required and still maintaining accuracy standards.
The experience of Topographical Surveys would seem to contradict the latter. We have found the graphical method quicker and we believe just as good where control is really sparse. We do believe, however, that although the absolute geographical position of a point is no better one way than the other, the mechanical adjustment of the slotted template method achieves a better relative scale over a small area, the main weakness observed being the tendency for the flights to bow, necessitating solid perimeter control.
In an effort to better understand the mechanics of the slotted template some very simple experiments were carried out by the Air Survey Section of Topographical Surveys. A set of perfect identical templates was prepared. All principal points were assumed to be in a straight line exactly 4" apart and pass points were made normal to the flight line at exactly 4". These templates were laid as a single row to mean scale, and the principal points at either end were pinned down. Forty templates were used in the assembly giving a span between the principal points of 39 bases of 4". It was found that the templates could be arched into a circular curve in either direction giving a total movement normal to the direction of flight of 1.04'. Further, by holding to mean scale at the middle and enlarging the scale in one direction and decreasing in the other direction, the centre was moved 0.41' towards the small scale end; apparently, then, with our equipment over a span of 13' the possible movement was 8% at right angles to and 6,4% along the direction of flight.
This bowing was considerably greater than experienced with graphical methods. Equivalent scale changes, however, are not uncommon. In an effort to introduce into slotted template work the directional strength of the graphical strip plot the bridging principle was tried, i.e., the direction of the principal point 2 photos distant was marked on every second photo.
This can be done as follows:-
Using the second of three successive photos, accurately transfer the principal points of the first and third photo to the middle photo. Join these image points carefully with a fine etched line and transfer image points along this line to the first and third photo; on the first and third photo draw a line radially from their principal points through these marks.
Cards were now prepared with normal pass points and with double base slots to carry in the principal point 2 photos distant.
Every second card was prepared in this manner and these were called "Azimuth Cards". The centre card or "Scale Card" initially used a false principal point, this point being the normal from the principal point to the base line joining the images of adjacent principal points. Later it became apparent that the scale photograph had no influence on the azimuth and consequently the true principal point could be used as the radial point when cutting slots, and further, this principal point could ride free in the assembly without disadvantage. Azimuth cards were, therefore, further modified as shown on the diagram below, no slot being left for the scale photo principal point.
Tests were now made over a 48" span using several combinations of normal and azimuth cards, the table below speaks for itself:
DOUBLE BOW EFFECT AT MIDDLE.
Movement normal to line of flight.
Ordinary Cards Azimuth Cards.
The peculiar result in the azimuth triple flight assembly is probably due to the decrease in elastic recovery with the increased mass of cards. A practical test of the azimuth template is currently in progress in Topographical Survey. Present indications are that perimeter flights control flights, and possibly every third or fourth interior flight will be assembled in this fashion, the rest of the flights being normal.
Another obvious advantage of this type of assembly is that initially only every second card need be laid, and the flight will have rigidity in the line of flight. This factor alone should simplify the assembly of the cards.