 Welcome to Caltrans LSIT LS exam preparation course. One aid in your preparation for California licensure examinations. A word of caution. Don't use this course as your only preparation. Devise and follow a regular schedule of study which begins months before the test. Work many problems in each area, not just those in this course's workbook, but problems from other sources as well. This course is funded by Caltrans, but you and I owe a profound thanks to others, the courses instructors from the academic community, the private sector, other public agencies, and from Caltrans as well. We wish you well in your study toward becoming a member of California's professional land surveying community. Hi, welcome to the photogrammetry unit of the Caltrans video training course. My name is Dick Burns. I'm chief of the headquarters photogrammetry section for Caltrans. In this training unit, we will be demonstrating or reviewing basic photogrammetry concepts as applied to surveying practice. Definition. Photogrammetry is the art, science, and technology of obtaining reliable information about physical objects and the environment through processes of recording, measuring, and interpreting photographic images and patterns of electromagnetic, radiant energy, and other phenomena. This definition is from the fourth edition manual of photogrammetry published by the American Society of Photogrammetry and Remote Sensing. Applications. In the field of land surveying, we are primarily concerned with the photogrammetric applications involving aerial photography and mapping, including the ground location of physical objects, lines, and points, and the determination of earthwork quantities. Course objectives. The objectives of this course are to acquaint the student with the basic principles of photogrammetry in order to gain the understanding to plan the execution of photogrammetric projects as required on the land surveyor's examination. This video will provide the first step in your understanding of the photogrammetric process. However, to be able to confidently solve the photogrammetry problems on the land surveyor's test, you will need considerably more study and practice. Now let's begin to examine the basic principles of photogrammetry. Number one, cameras. The aerial camera is a vital part of the photogrammetric mapping process. The metric quality aerial cameras used for photogrammetric mapping are very precise instruments with virtually no distortion from the object space, ground to the image space, photograph. Here we see the camera being loaded and mounted in the aircraft by the pilot and aerial photographer. The mapping cameras come in several focal lengths. Four common focal lengths are three and a half inch considered in photogrammetry to be a super wide angle, six inch wide angle, eight and a quarter inch normal angle, and the 12 inch narrow angle with a six inch camera by far the most widely used. Most of the aerial cameras currently in use are manufactured in Switzerland and Germany and new cameras are priced in the $300,000 range. Focal length. The focal length of a camera affects the scale of the photography for a given flying height and the angle of coverage of the photograph. The three and a half inch and six inch cameras would be considered as wide angle lenses in a commonly used 35 millimeter camera while the eight and a quarter inch would be the normal lens and the 12 inch would be more like a telephoto. As with handheld 35 millimeter cameras, the longer the focal length, the greater the magnification and the smaller the field of view. Conversely, the shorter focal length yields less magnification and a wider viewing angle. In this graphic, we see the comparison of the photo scales of six and 12 inch cameras flown from the same flying height. The 12 inch camera yields a scale twice as large as a six inch camera and covers one quarter of the area. Here we see the comparison between the eight and a quarter and three and a half inch cameras. Note that the three and a half inch camera covers a large area at a small scale similar to a 35 millimeter wide angle lens. Flying height focal length relationship. Here we see four cameras flying at different altitudes but photographing at the same scale. Compared to a six inch focal length camera, a longer focal length lens will allow a higher flying height and a shorter focal length lens will allow a lower flying height, all yielding the same photo scale. In this presentation, we will direct your attention to the six inch focal length camera due to its extensive use, but you should be aware of the capabilities of other mapping cameras. Format. The cameras we will be concerned with in this presentation will have a nine inch by nine inch format. All aerial mapping cameras have fiducial marks, usually eight around the perimeter of the camera frame, which appear in the photographs as small white measurement points. The main purposes of the fiducial marks are to define the optical center of the camera lens and to check for film distortion and film flatness in the focal plane. The cameras are calibrated by the US Geological Survey for lens distortion, fiducial mark coordinates, model flatness, and other parameters. Number two, scale. Map and photo scales are expressed in various forms, such as one inch equals 250 feet, one to 3,000, one inch over 250 feet, 100 scale, or one inch equals four miles. These various forms of scale represent units on the map or photograph in relation to units on the ground. The large scale, small scale relationship can be confusing. For example, one inch equals 50 feet is a larger scale than one inch equals 100 feet. Some methods to visualize and remember this relationship are that a building, as shown here, would appear larger on a 50 scale map than the same building would on a 100 scale map. Or that the ratio one to 600, representing a 50 scale map, is larger than one to 1200 for a 100 scale map. To plan a photogrammetric project and check the photography for usability on the project, the scale relationship must be understood. Basic formulas, S equals F over H minus small H, where S equals scale, F equals focal length of the camera, H equals flying height above sea level, and small H is the elevation of the mean terrain. This is the basic scale formula, and it is derived as shown on this diagram. Small D is a distance on the photograph, and D is a corresponding distance on the ground. By corresponding sides of similar triangles being proportional, small D over D equals F over H minus small H. Or since small D over D equals scale, then S equals F over H minus small H. Scale is also expressed as a denominator with one as a numerator, such as one over 3000, a ratio, one of any unit on the map or photograph equals 3000 of the same units on the ground, or with units such as one inch over 250 feet. Frequently, we know the scale and need to solve for the flying height above sea level, H. Therefore, we can convert the basic scale formula as in the middle equation in the graphic to one over X equals F over H minus small H with X representing the scale denominator with one as the numerator and solve for H. Cross multiplying, we have H minus small H equals X times F. Solving for H, we have H equals X times F plus small H. Now let's take time to look at the diagram again and see the scale can be in these three forms. S equals F over H minus small H equals small D over D and equals when the numerator is reduced to unity one over X. When the scale is reduced to unity one over X is actually the reciprocal of small D over D. Although it is not shown on the graphic F is often stated in millimeters so the number of millimeters per inch, 25.4 is a number which should be among those committed to memory. Again, remember that S can be in these three forms. F over H minus small H small D over D and one over X. Now we'll take a few seconds to absorb these relationships. Number three, flying height. When planning a photogrammetric project the flying height may be computed by returning to the basic scale formula or solving directly for H as shown above. For example, to photograph at a scale of 1 inch equals 250 feet with a 6 inch focal length camera over terrain with a mean elevation of 1200 feet what is the required flying height above sea level, H? We can start at the beginning basic formula and go through all the steps using these numbers or go to the bottom formula and solve directly for H. Let's do that. H equals X times F plus small H or H equals 250 times 6 which is 1500 plus 1200 the mean terrain elevation so H equals 2700 feet. Now we can also solve the equation by converting all the numbers to feet. If 1 inch equals 250 feet then 1 foot on the photo will equal 12 times 250 or 3000 feet on the ground. Here the scale is a ratio of 1 to 3000. The 6 inch focal length will convert to 0.5 feet. The equation will be H equals X times F plus small H which is H equals 3000 feet H equals 3000 times 0.5 plus 1200 or H equals 1500 plus 1200 and again H equals 2700 feet. To verify that the photography was flown at the correct altitude for the project again solve for H. If the photography has pre-marks with known coordinates divide the ground distance in feet d by the photo distance in inches, small d. This is the reciprocal of small d over d reducing the photo scale to unity and determining X. Then multiply the result by the focal length in inches and add the mean pre-mark elevation. The result of these calculations is the flying height above sea level. If the control values for the photograph are not available at the time of checking the ground distance between features on the photo can be determined from a quad map. Terrain height, terrain height, small h is the mean elevation of the mapping area along the flight line. Terrain height is normally determined from a USGS quad map or other contour maps. Sea factor. Sea factor is the flying height above the mean terrain divided by the contour interval H minus small h over the contour interval. Sea factor is the formula to determine the accuracy of the entire photogrammetric system including the flying height, the camera, the film and film processing, the diapositive and diapositive processing, the photogrammetric mapping instrument and the instrument operator. 1500 is a conservative maximum sea factor. This means with the flying height above mean terrain of 1500, a one foot contour map may be produced or at 3000 feet a two foot contour map. Some photogrammetry firms have stated much higher sea factors up to and above 2500. These claims are made for economic reasons such as fly higher, less ground control and fewer models etc. These are all strong incentives to state a higher sea factor than can be realistically achieved and the use of sea factors higher than 1500 must be considered on that basis. Flying tolerances. An aircraft on an aerial photography mission cannot fly exactly at the planned flight line at the planned altitude and must be given a reasonable amount of allowable error. Caltrans uses one half inch at photoscale on either side of the flight line for horizontal tolerance and five percent of the flying height at scales up to and including one inch equal 400 feet and three percent of the flying height at scales smaller than one inch equals 400 feet for vertical tolerance. Overlap. Mapping photography is normally planned at 60 percent overlap. 60 percent overlap means that each successive photograph overlaps the photo before it by 60 percent. If there is 60 percent overlap there is 40 percent gain new area photographed with each successive exposure along the flight line. An aerial photograph is nine inches. 40 percent of nine inches is 3.6 inches. Therefore, nine inch by nine inch photography with 60 percent overlap has a gain of 3.6 inches on the photo regardless of scale. Here we see the overlap from a side view. Caltrans overlap tolerance is from 55 to 65 percent. Number five, side lap. Side lap is the area of side overlap between two adjacent flight lines and is calculated the same way as overlap. Side lap is normally planned within a range of 25 to 40 percent. Here we see the side lap concept from an in view. Number six, model concept. A model is defined as the area along the line of flight between centers of two overlapping aerial photographs and bounded parallel to the flight line on each side by a line one inch from the edges of the photograph. This is often referred to as a neat model. The normal size for a neat model is 3.6 inches along the line of flight with a seven inch width. For flight planning, this is reduced to six inches to allow for horizontal flying tolerance. The length of a neat model will be changed due to a variation in overlap and the width may be reduced due to crab, the camera not being square to the flight line in one or both photos. Stereo vision. Stereo vision is achieved by the left eye viewing the left photograph of a model and the right eye viewing the right photograph. The ability to see the same images from different angles with each eye enables three-dimensional viewing or stereo vision. When viewing the model, the operator sees a measuring or floating mark which appears to be floating above, on, or digging into the ground. The floating mark is used as the point of measurement in all three axes, much the same as surveying instrument crosshairs. Models and flight lines. The concept of a model can be confusing when planning aerial photography because the first model doesn't begin until the center of the first photograph in the flight line. This means that the entire first half of the first photo and the last half of the last photo in the flight line cannot be used for mapping because these images appear in only one photograph and will not allow stereo vision. In this diagram, we can see the relationship of the photography to the flight line. We can only map an area from the center of the first photo to the center of the last photo in a flight line. We can also see that it always takes one more photograph than models to map a given area along the flight line. In this diagram, we see the flight line shown as models rather than photographs. This is the form used for flight planning purposes. Flight path. A flight crew usually consists of a pilot and an aerial photographer. To fly a flight line, the pilot selects landmarks from the map to orient the aircraft and selects the correct altitude using the altimeter. A dry run is often made to make sure the flight line is correct. On the photo run, the photographer is responsible for the operation of the camera and keeps the pilot correctly aligned. The photographs are taken at the proper intervals to ensure correct overlap by a mechanism in the camera. At one inch equals 250 feet photo scale, the aircraft is flying at 1,500 feet above the terrain, taking photos at 900-foot intervals. When the flight line, into the flight line is reached, the pilot will turn the aircraft in two 180-degree turns and align with the next flight line. Models in blocks. Blocks consist of parallel or nearly parallel flight lines. The photography from parallel flight lines must have side lap in order to be usable and cannot have any gaps. The amount of side lap usually ranges from 25 to 40 percent. To calculate the photo distance between flight lines, subtract the side lap percentage from 100 percent and multiply by 9 the photo size. For example, 25 percent side lap, 100 percent minus 25 percent equals 0.75 times 9 inches equals 6.75 inches between flight lines. Number seven, photo control. A well-controlled need model requires the following points. Three well-distributed horizontal control points to set the model scale. Occasionally, two horizontal control points are permissible, but the third provides a check. One vertical control point located at each of the four corners of the need model for leveling. These points are commonly referred to as wing points. The photo control for a mapping project is normally marked by placing cloth or painted targets over the ground control points prior to aerial photography. This process is called pre-marking and the targets are called pre-marks. It is possible to identify the control points after the photography is taken. This process is called photo identification or post-identification and is less certain and accurate than pre-marking. Pre-marking is the preferred method of photo control. Aerotrangulation. Aerotrangulation is a computerized procedure used to reduce the number of pre-marked field surveyed control points necessary to map a project. Every model requires four wing points to level and two or three horizontal points to scale it. Using aerotrangulation, some of the points are artificial or pug points marked on the photos with mathematically derived positions and elevations instead of being field surveyed. Virtually all of Caltrans mapping is done with aerotrangulated control. Mapping control requirements vary somewhat with the organization's mapping system and the use of maps. At Caltrans, we are concerned with the vertical accuracy along the roadway and our standards are conservative in that respect. That is, more vertical control along the flight line. Caltrans control requirements for 1 inch equals 50 feet mapping using aerotrangulation are a pre-mark with horizontal control in each fourth or fifth model beginning and ending outside the mapping limits. No extrapolation. A minimum of three horizontal points in each flight line are required including points common to other flights. Pre-marks with vertical control along the wing point lines outside the mapping limits at approximately 1,800 foot intervals on alternate sides of the models. Pre-marks with vertical control in the corners of each flight line. Pre-marks with vertical control at 450 foot intervals within 250 feet of the flight line. Common horizontal and vertical pre-marks may be used where two or more flight lines join to save field costs. Common horizontal and vertical pre-marks may also be utilized between parallel flight lines. Block photo control. Block photo control requirements seem to vary considerably with the organization and the individual control planner. There is one area of agreement, however, and it is that the perimeter of the block should be well controlled with no extrapolation. The graphic shows a typical example of block photo control. Caltrans, because we are mainly concerned with highways, does more single flight line mapping than block mapping. Pre-marking. Pre-mark requirements. A symmetrical pattern around the photo control point is one of the prime requisites for a photogrammetric pre-mark. For large-scale mapping, it is important to have a separation between the center point of the pre-mark and the legs. This configuration allows a discrete point to be read in the photogrammetric instruments rather than guessing at the intersection of a solid cross. Cloth pre-marks are normally used on natural ground and painted pre-marks with the same dimensions and a dull black background are used on pavement. Cloth pre-marks may also be used on asphalt pavement or traffic as light. Caltrans pre-mark requirements for various scales are shown in the quick reference table in your workbook. Pre-mark placement. Pre-marks must be exactly centered over the point they identify to maintain scaling accuracy. Pre-marks are read to a least-round reading of a hundredth of a foot in an analytical plotter using 1-inch equals 250-foot photography. All pre-marks should be placed in positions which will ensure their appearance and the models there to control. A pre-mark which appears in only one photo is useless. Pre-marks should be placed where they are visible in the photos they control in the sun at the time of photography and checked to make sure they are in place immediately prior to photography. Accuracy specifications. Caltrans uses the photogrammetric mapping accuracy outline in the manual of photogrammetry under Photogrammetry for Highways Committee 1968. Simply, these accuracy specifications state that 90% of the contours shall be within a half-foot contour interval and all within one contour interval. 90% of the well-defined planimetric features shall be within 1-40th of an inch and all within 1-20th of an inch. And 90% of the spot elevation shall be within a quarter-contour interval and all within a half-a-contour interval. The accuracy of photogrammetric work is based on field surveys which presumably have no discernible error. There is another similar set of specifications for general large-scale photogrammetric maps in the manual of photogrammetry fourth edition. Both versions of these specifications are widely used. Number nine, mapping. Instruments. There are two general types of photogrammetric plotters used for mapping. The analog plotters, which recreate the photogrammetric model by positioning the photographs in the same-scale position with the same tilts existing at the time of exposure, thus creating the model to a smaller scale. The newer type of plotters, called analytic plotters, are computer-assisted and create the model by positioning the photographs in a plane and using the computer to simulate the tilts of the photography in space by computer-controlled incremental movements in the X and Y directions. There are many photogrammetric instruments available for mapping purposes. In California, the most widely used analog plotters are Wild A10, B8, Santoni 2C, Galileo G6, G8, Zeiss Planckart, Zeiss Jainatopokart. The most widely used analytical plotters are the Zeiss C120 and P3, the Kern DSR11 and DSR14, and the Wild or Leica BC1, BC2, and BC3. Base sheets. Base sheets are the blank maps which control the control and the grid ticks are plotted to provide the horizontal and vertical datums to set up the models to begin plotting the maps. These maps provide the points which are located on the photos as pre-marks or pugs and plotted in the correct coordinate position on the base sheet to allow the plotter operator to orient the model to the ground both horizontally and vertically and draw the map. Base sheets are normally five-time enlargements of the nominal photography scale. Diapositives. Diapositives are a positive print of the aerial photographs on either glass or mylar. Diapositives are printed on transparent materials in order to use them on photogrammetric plotters. The pugs are drilled in the photographic emulsion of the diapositives. Table enlargements. Photogrammetric mapping is usually a five-time enlargement of the aerial photography. However, with an automatic table, the enlargement ratio can be set for an enlargement of up to 20 times. A panor pencil of the table follows the movement of the plotter measuring mark at the set enlargement ratio thus drawing the map. Computer-aided drafting and design or CAD mapping. Photogrammetric plotters may work in conjunction with the mapping system to draw the map directly into the computer instead of making a hard copy. Using conventional methods a manuscript is created on the plotter and then finished drafted and finally digitized into the computer. The direct process avoids the map drafting and map digitizing steps to create the CAD map. This is played on the screen on an editing workstation. Photogrammetric project tasks. Let's go over the steps necessary to complete a photogrammetric project. Number one, mapping limits. The first step of the photogrammetric mapping process is the determination of the mapping limits. The mapping limits are usually determined by the engineer requesting the mapping. Number two, required scale and contour interval. The engineer will usually request the scale and contour interval needed for the map. The scale and contour interval is somewhat fixed by the proposed use of the map and the terrain. Number three, flight plan. The flight plan gives the length and direction of the flight lines and the altitudes above sea level for the photographer to photograph the project. These are templates showing the models at 6 inch mapping width. These templates are made for several mapping scales and are made to be used on a 1 inch equals a thousand feet enlargement of a USGS quad map. The simple template greatly eases the flight planning process. Number four, control plan. The control plan shows the surveyors where to place the field control which through error triangulation will provide the mapping control. This is the completed flight and control plan. Number five, place pre-marks. The pre-marks are placed over the control using either cloth or paint. They must be precisely placed over the point and maintained so they're visible at the time of photography. Number six, survey pre-marks. The horizontal and vertical or vertical only values for the photo control are tied from the project control. Good surveying procedures must be used. All points must be turned through or have an independent check. Surveys do not have to be completed before the aerial photography but the pre-marks must be in place. Number seven, aerial photography. Aerial photography should be ordered from the project site to ensure that the weather is clear in the area to be photographed. Good aerial photography requires a great deal of skill to accomplish but is done in a short period of time in relation to the other tasks. Number eight, survey calculations. Survey notes must be reduced to coordinates and elevations of the photo control. Number nine, check aerial photography. The aerial photography is checked for coverage, overlap, flying height, tilt, image motion and pictorial quality. Number ten, aerotrangulation. Aerotrangulation densifies the skeleton ground control to provide sufficient photo control to level and scale the models. Number eleven, base sheets. Base sheets are the blank maps with only the field and aerotrangulated control plus the grid ticks plotted. The control is used to scale and level the models and the maps are compiled on the base sheets. Number twelve, map compilation. The maps are compiled one model at a time, usually on three model sheets or into a CAD file. To compile planimetry, the operator traces the feature with the measuring mark on the surface of the feature. For contours, the operator sits the elevation of the contour in the plotter and traces the contour with the measuring mark just covering the ground. Number thirteen, map checking. Maps should be independently checked by either photogrammetric or field methods for conformance to specifications and correct mapping symbology. Map corrections. If the maps do not meet specification, then corrections are required. Number fifteen, map acceptance. The final maps are accepted when the required corrections are completed. These are the fifteen steps photogrammetric mapping normally follows in a production environment. Now let's work the photogrammetry problem on the nineteen ninety one land surveyors test. Problem statement. Your client owns sections nine and sixteen and the masterly four thousand feet of sections ten and fifteen township four south range twenty three west San Bernardino base and meridian. You have been asked to provide horizontal and vertical control for the topographic mapping that is to be used for planning purposes. Vertical photography taken with a six inch focal length camera on a nine by nine inch focal plane is to be used. Analytical bridging arrow triangulation is not to be considered. The following factors control the project make no assumptions. Number one a five foot contour interval is required. Number two model size is three point six inches by seven inches for a single flight line and three point six inches by six point three inches for two or more adjacent flight lines. Number three the C factor to be used for this project is eighteen hundred. Number four the map is to be compiled at a five to one ratio. Number five the average terrain elevation is twenty five hundred feet above sea level. Number six the minimum target size to be used for pre-marking on the ground is not to be less than a thousandth of an inch by a hundredth of an inch at the photo scale. Number seven per a recent record of survey each section has been found to be of standard dimensions. Problem requirements number one based on the above specification determine the following show all work A the minimum number of flight lines required three points B the required flying height above sea level three points C the minimum number of models required three points D the minimum number of photographs required three points E the minimum number of horizontal and vertical control stations were required to provide adequate checks five points F the negative scale three points G the nominal map scale three points H the minimum length and width of the target placed on the ground as a pre-mark four points Number two give the accuracy requirements for each of the following based on the requirements of the national map accuracy standards A contours one point B spot elevations one point C planametric features one point problem solution let's start with the determination of the mapping limits which are one mile plus four thousand feet in the easterly direction and two miles in the northerly direction so the mapping limits are ninety two hundred and eighty feet by ten thousand five hundred and sixty feet now we have to determine the answer to problem one B the flying height above sea level using what we have to work with the contour interval five feet and the C factor eighteen hundred the C factor equals H minus small h over the contour interval when we insert the given numbers we have eighteen hundred equals H minus twenty five hundred over five or solving for H H minus twenty five hundred equals nine thousand or H equals eleven thousand five hundred feet then H minus small h equals nine thousand feet now we know the answer to one B H is eleven thousand five hundred feet since H minus small h equals nine thousand we can determine the answer to one F by using the scale formula S equals F over H minus small h or S equals six inches over nine thousand feet or one over X equals six over nine thousand or six X equals nine thousand or X equals fifteen hundred the answer to one F the negative scale is one inch equals fifteen hundred feet or as a ratio one to eighteen thousand we were given that the map is a five time enlargement of the negative scale therefore one over fifteen hundred five equals one over three hundred so the answer to one G the nominal map scale is one inch equals three hundred feet or one to thirty six hundred now let's look at the area we can map with one model which has a three point six inch by seven inch size three point six times fifteen hundred equals fifty four hundred feet and seven times fifteen hundred equals ten thousand five hundred feet so one model will map an area fifty four hundred feet by ten thousand five hundred feet so two models flown in the north south direction will cover an area of ten thousand eight hundred feet in the north south direction and ten thousand five hundred feet in the east west direction adequately covering our mapping limits of ten thousand five hundred and sixty north south and ninety two eighty feet east west the answer to one A is one flight line the answer to one C is two models and since there is one more photo than models per flight line the answer to one D is three photographs we need to answer one E the minimum number of horizontal and vertical control points to provide adequate checks three vertical control points form a plane so that that is the minimum number to level a model and the fourth provides a check to scale a model requires a minimum of two points and the third provides a check let's look at the little diagram at the top of this graphic in the first model at the bottom of the little diagram there are three horizontal control points and four vertical control points but two of the horizontal and two of the vertical control points will also control the second model for the second model we need to add one horizontal and two vertical points for a total of four horizontal vertical this is the grading plan answer to one E the planners at Caltrans would have had vertical control on the four horizontal control points and vertical only on the remaining two points the real world answer as shown on the little diagram would have been four points with horizontal and vertical control and two vertical only points this answer was not in the grading plan but may have held up under protest problem one H as for the minimum length and width of the target placed on the ground as a pre-mark with given dimensions on the photos since the photo dimension is ten times as long as it is wide and pre-marks are usually crosses this might be better described the minimum length and width of a leg of the target rather than the length and width of the target anyway the photo scale is one inch equals fifteen hundred feet so a thousandth of an inch would be one point five feet and a hundredth of an inch would be fifteen feet problems two A two B and two C as for accuracy requirements contours, spot elevations and planometric features respectively based on the national map accuracy standards here there is a problem with the grading plan answers the answers are contours ninety percent within a half a contour interval or two and a half feet spot elevations ninety percent within a quarter contour interval or one point two five feet and planimetry ninety percent within a thirtieth of an inch or ten feet two A and two B grading plan answers appear to be from the reference guide outline the photogrammetry for highways committee nineteen sixty eight as stated in the fourth edition of the manual of photogrammetry answer two C appears to be from the general national map accuracy standards for publication scales larger than one to twenty thousand the old third edition of the manual of photogrammetry does have accuracy standards for highways divided into two classes one for forty fifty one hundred and two hundred feet to the inch and the other for four hundred five hundred eight hundred and a thousand feet to the inch the grading plan answers match the ladder but the three hundred scale photography from the problem doesn't match either set of standards the grading plan answers don't seem to fit any of the accuracy specifications in the final analysis this is a good practical photogrammetry problem with a couple of areas which may require protests this is not unusual most of the photogrammetry problems do have ambiguous parts to the questions this is sometimes because there are different planning variations which will still meet specifications and sometimes they're just plain wrong your best defense is to study and understand the basic principles read the questions carefully and use calculations based on the problem information even if it's wrong or impractical for gross errors or emissions make assumptions in writing and be prepared to protest show your work be careful some problems are tricky even for us old photogrammetries use your workbook plus work the problems from old exams get comfortable with basic photogrammetry it's not that difficult there are some relatively easy photogrammetry points waiting for those who are prepared good luck