The king Aeson, being tired of the cares of government, surrendered his crown to his brother Pelias on condition that he should hold it only during the minority of Jason, the son of Aeson. When Jason was grown up and came to demand the crown from his uncle, Pelias pretended to be willing to yield it, but at the same time suggested to the young man the glorious adventure of going in quest of the Golden Fleece, which it was well known was in the kingdom of Colchis, and was, as Pelias pretended, the rightful property of their family. Jason was pleased with the thought and forthwith made preparations for the expedition. At that time the only species of navigation known to the Greeks consisted of small boats or canoes hollowed out from trunks of trees, so that when Jason employed Argus to build him a vessel capable of containing fifty men, it was considered a gigantic undertaking. It was accomplished, however, and the vessel named "Argo", from the name of the builder. Jason sent his invitation to all the adventurous young men of Greece, and soon found himself at the head of a band of bold youths, many of whom afterwards were renowned among the heroes and demigods of Greece. Hercules, Theseus, Orpheus, and Nestor were among them. They are called the Argonauts, from the name of their vessel. The "Argo" with her crew of heroes left Thessaly and having touched at the Island of Lemnos, thence crossed to Mysia and thence to Thrace. Here they found the sage Phineus, he who had mapped the stars in the heaven, and the ends of the Earth, and from him he received a map as to their future course. It seems the entrance of the Euxine Sea was impeded by two small rocky islands, which floated on the surface, and in their tossings and heavings occasionally came together, crushing and grinding to atoms any object that might be caught between them. They were called the Symplegades, or Clashing Islands. Phineus instructed the Argonauts how to pass this dangerous strait. When they reached the islands they let go a dove, which took her way between the rocks, and passed in safety, only losing some feathers of her tail. Jason and his men seized the favorable moment of the rebound, plied their oars with vigor, and passed safe through, though the islands closed behind them, and actually grazed their stern. They now rowed along the shore till they arrived at the eastern end of the sea, and landed at the kingdom of Colchis. Jason made known his message to the Colchian king, Aetes, who consented to give up the golden fleece if Jason would yoke to the plough two fire breathing bulls with brazen feet, and sow the teeth of the dragon which Cadmus had slain, and from which it was well known that a crop of armed men would spring up, who would turn their weapons against their producer. Jason accepted the conditions, and a time was set for making the experiment. Previously, however, he found means to plead his cause to Medea, daughter of the king. He promised her marriage, and as they stood before the altar of Hecate, called the goddess to witness his oath. Medea yielded, and by her aid, for she was a potent sorceress, he was furnished with a charm, by which he could encounter safely the breath of the fire breathing bulls and the weapons of the armed men. At the time appointed, the people assembled at the grove of Mars (Ares), and the king assumed his royal seat, while the multitude covered the hill sides. The brazen footed bulls rushed in, breathing fire from their nostrils that burned up the herbage as they passed. The sound was like the roar of a furnace, and the smoke like that of water upon quicklime. Jason advanced boldly to meet them. His friends, the chosen heroes of Greece, trembled to behold him. Regardless of the burning breath, he soothed their rage with his voice, patted their necks with fearless hand, and adroitly slipped over them the yoke, and compelled them to drag the plough. The Colchians were amazed; the Greeks shouted for joy.

Our intent with this pamphlet is to present in simple format the properties involved in photogrammetry, a division of the engineering sciences. This pamphlet is not meant to be a textbook of the science, or a complete work, but is intended to inform the layman of the principles involved so he or she may converse with the professional. It is recommended that if you are interested in a greater understanding of this field, you obtain a published text covering the subject. Some illustrations used in this book are from (PHOTO GEOLOGY by Victor Miller published by McGraw-Hill, 1961). This is an excellent source of information on photogrammetry without engineering referencing. When you select a project for mapping, your one goal should be to obtain a product that fulfills your greatest need. If you are unfamiliar with all the requirements and capabilities of mapping, find someone who has been through the process. Consult with your engineering department, whether contracted or in house, and explain what you wish to accomplish. If you are looking for simply a development planning tool, then your requirements are not as critical as those of an engineer. Most GIS systems are set up for planning and therefore are not of an accuracy that can be used by engineers. Where planning looks at the overall picture, engineering looks at the specific. It is commonly thought one system can fit all, but this is not the case. There are restrictions with file size and data stored; however as the computer systems continue to develop we see these limitations becoming less of a problem.

This diagram depicts the standard configuration of a camera. It could be a cardboard camera or the most expensive on the market. The principles are the same. f = focal length It is measured from the center of the lens to the surface of the negative.

The two diagrams above depict how photos relate to one another. The top diagram demonstrates parallel flights over an area. In order to allow for accurate measurements a side overlap must be maintained of 30% on most ground. But where the ground is very mountainous the side lap must be increased up to 50%. In low level flights closer to the terrain, the objects being photographed show the greatest distortions. The same consideration must be given to forward flight path overlap as depicted in the second diagram. On flat ground 60% overlap will work quite well, but again in very mountainous terrain the forward overlap must be increased up to 85%.

In this diagram and the next we can see the effect that differing heights of terrain have on the photo scale. Those areas closest to the camera (on the right) appear larger than they actually are and they cover less ground. The areas further from the camera (the center) appear smaller and cover a great deal more area. An orthophoto (ortho) changes the scale of the photo throughout so that a measurement can be made on the ortho with relative assurance of true distance. In the beginning of orthophotography machines were used to accomplish this task. A small slit in the table allowed a portion of the projected image to expose the underlying photo sensitive material. The operator would raise or lower the table as needed to keep the slit at ground level. With the advent of computers, orthophotography became much more assessable and easier to produce as no single person had to tend a machine for long, tedious hours. Computers have increased in speed and storage to allow layering of mapping and orthophotos on the same plot. Plotters have also seen a dramatic revolution. Near photo quality plotters are affordable by most mapping offices.

An ortho of this diagram would scale the center portion up until the tiles were the same size and area as those on the left and reduce the tiles on the right to the size of the newly scaled center portion. This would make all the tiles the same size. The perimeter of the photograph would no longer be a square; the center portion would extend by about a square on both top and bottom. The right side would be reduced in length by about a square, both top and bottom.

The two preceding pages of diagrams show the effect of air base (air base is like the distance between your eyes). The air base of camera positions is a direct result of focal length. The shorter the focal length, the greater the air base. In using a standard hand-held personal camera, you can zoom in on an object you wish to view at close range. This zooming in increases the focal length of the camera. By zooming out, you shorten the focal length of the camera, things appear further from you, and a much greater area is covered.. If you use a fisheye lens, you can see 180 degrees because the fisheye lens is half a sphere. Focal length is critical in mapping and needs to be specified for each project to obtain the best results. The most common focal lengths are 3.5 inch, 4 inch, 6 inch, 8.25 inch, 12 inch and 24 inch. Using the human eye as the base for all reference, the 12 inch lens is a duplicate or a 1:1 ratio. The 6 inch camera, the most frequently used in the industry, fulfills two requirements. First, it allows photography to be taken at only half the height, and second, it exaggerates the height to twice that of the 12 inch thereby allowing a more precise measurement of the height. Conversely, the 24 inch camera reduces the height to half of what appears in the 12 inch lens. If the ground is extremely flat, you need to exaggerate elevation even more. This exaggeration would require the 3.5 inch lens. Other factors that dictate lens length are vegetation and buildings. Timber stands and tall buildings block out the view of the ground in shorter focal length lens because of the exaggerated parallax. For this reason the forest industry requires 8.25 inch or 12 inch cameras so the ground can be viewed through the stands of trees. A standard pair of stereoscopes shorten the focal length. The end result is a doubling of the image size. To enlarge the image size even further, a lens terrain is needed to shorten the focal length and to allow viewing. Light and shadow are very important in photography. The common belief that a bright sunny day is best depends on what you wish to achieve. Too bright a sun will cause sun spots (a point where through an effect called refraction, the sun light condenses around the aircraft, to a point on the ground where the shadow strikes) which cannot be removed from the image by dodging. While a high overcast tends to dull the image, it also reduces the contrast and limits the sun spots to a controllable factor. The reduced shadow allows viewing in the shaded areas. The instrument on the next page is a Kelsh stereo plotter, a fixed five time projection enlargement machine. It was the workhorse of the industry for many years, and became the standard by which other stereo plotters were judged. The C factor of the machine was 1:1500. If you were flying at an elevation of 3000 feet with a 6 inch camera, you could expect to produce a two foot contour: 3000/1500 = 2 CI. The width of a 5 mm pencil lead at 1" = 100' is 2 feet. A spot elevation could be read to 1/4 of the CI, in this case 0.5 feet, or you could measure repeatedly on a well adjusted machine to 0.005 inch. As lens terrain machines became more prevalent and their price more affordable, companies began shifting to this type of machine. As a whole, the C factor did not change. A more precise version was soon produced with a C factor of 1:2000. This new version allowed for an increase in flight height, but there were some other tradeoffs that were not so good. The Kelsh required four vertical and three horizontal ground control points. The new machines, called heavy plotters because of their construction, allowed operators to carry forward several points usually recognizable images ( power poles, sidewalk corners, some times points pricked into the image) along the neet model line( the imaginary line across from side to side trough the photo center) called `pass points', so fewer and fewer control points were used. These machines also allowed bridging by using a method called, `base-in-base-out', where one plate is adjusted to the other. This bridging method had been reserved for special machines in the past which were multiplexed or had several projection systems on the same stereo plotter.

With the advent of computers, everything changed. No longer was bridging carried out on machines. Now analytical software handles bridging through mathematical solutions, and measurements are no longer a function of ray rods or projections, but are now measured as positions on the flat photograph. Also vertical measurements are a mathematically computed position. Two of the greatest benefits are that photo centers are location precisely accuracy has became a function of scaling in the X and Y position. The first plotters of this kind were slow and you could see the stages move and adjust to one another. The next several generations of computers made this motion more fluid. A French company named Matra offered the first off the shelf model for the industry. It was expensive but accurate and had a C factor of 1:4000. Reece Jenson of Matra Technology, Inc in the U.S. constructed a more affordable version with the more common C factor of 1:2000. The common belief is that the C factor does not matter anymore; however it is still very much a part of photogrammetry. Map scales are tied to the photo scale. Good accurate mapping should not be produced at a scale greater than six times the photo scale. If photography is taken at a scale of 1" = 600' and mapped in a machine with a C factor of 1:2000, the best vertical point you can expect is 600 x 6 = 3600 flight height, 3600/2000 = 1.8' contour interval, and 1.8/4 = 0.45' spot elevation height. Therefore, you cannot measure a curb that is 0.5'. Control patterns that were developed for the multiplexed and heavy plotters are still relevant today. A good practice is to start and end your flight lines with three well placed control points and to have one point in every other model. If your flight line is greater in length than five models (closer to ten or more), then you should control approximately every fifth model with three control points. The only exception is when the flight you are working on is part of a block of several photo flights. Analytical solutions are based on either of two methods. The model tie point method assumes that each model is plumb to the center of the earth and that the points along the tie are fixed in position. However, in this method the photo center moves position to accommodate the plumb position of the model. The second method assumes that the photo centers are in a fixed position and the model tie points are adjusted to achieve a level plane in each model. GPS control on photo centers uses the second method and can be used on large projects with minimal ground control. This offers good cost savings as well as expedient production. However, check points should be placed throughout the mapping site. An accuracy test should be performed in the field by profiling from one known point to another known point. Statistical accuracy is all right for statistics, but in the case of mapping you are interested in knowing where the true ground is and how the map portrays it.

	Chip Westbrook
	Consulting Photogrammetrist