| Photogrammetric Mapping Connects with CAD
In the early '80s the photogrammetrist was already making strides into the world of CAD. In the '90s the trend continues.
By Dennis Shimer The problem is a typical one, as a community grows roads and other crucial infrastructure are taxed to their limit, carrying loads that were never imagined when they were first designed. The engineer needs an accurate record of what now exists in a format that will allow him to inventory, evaluate, and redesign using his state of the art engineering system. As is often the case it is determined that the most efficient way to gather the raw data is by mapping the site using traditional photogrammetric methods from precision aerial photography. A recent Central Ohio project offers a good example of how Computer Assisted Drafting (CAD) can add to the speed and accuracy with which a photogrammetrist can collect the necessary mapping data, while providing greater flexibility to the final product. The Project
The city of Hilliard-home of the corporate offices for CompuServe-has been one of the fastest growing municipalities in the state of Ohio for several years. Many roads in and around the city bear the burden of heavy residential and commercial growth. To ease the load along a five mile stretch of Rome-Hilliard Road, city officials elected to implement a utility upgrade and road widening project. M.A.N. Mapping Services Inc. was chosen to map the existing road and provide the data to the engineers.
All base maps were taken from photography exposed at a scale of 1:2400 with photo control accomplished using traditional ground survey methods which was then densified using aerotriangulation. This data was stored with text and symbolization appropriate for an output scale of 1:200. The ground was stored as a DTM with a one half meter contour interval generated from it. The product delivered included reduced scale (1:500) paper check plots, a topo mapping file matching the check plots in Autocad release 12 format, and a separate DTM file translated to match the clients specifications for their Eagle Point engineering package. History
To fully appreciate just how far mapping has come to get to the digital data that regularly appears on your terminal, we need to take a very brief tour of the past. Brief in that one needs only to return to 1980 to go back to a time when the majority of topographic mapping was being transferred directly from the stereo instrument to paper via gear driven tables or pantographs. These manuscripts were generally produced in a light pencil and were then inked or scribed away from the instrument. There was an inevitable amount of generalization involved in interpreting the pencil scratching that became the final map. Often it was not the stereo operator who did the final drafting, providing another source of potential error. While these manuscripts were generally more artistic than their digital counterparts they were often as different as the people who did the final edit or "heavy up." They were also only as stable or lasting as the media they were drawn on, and could not be easily replicated or replaced.
In the early '80s the photogrammetrist was already making strides into the world of CAD. These early forays were often taken on cumbersome unreliable computers which predated the PC-simply appliances to store the unedited data for future reference. Editing was so cumbersome as to be nonexistent for all but the worst blunders. Data transfer was almost unheard of because there were so few systems to even transfer anything to. Data structures were proprietary, simple, and often frustrating.
At the same time the mapping community was beginning to experiment with the idea of storing the shape of the ground surface as a digital terrain model (DTM), already recognizing the tremendous amounts of data generated by smooth contour lines.
With the introduction of the personal computer and the growth of CAD in other disciplines, the mapper was expected to understand his own system, his clients system and how to transfer data between them. This would include the ability to customize the data to meet what is often a variety of needs, even on a single project. It is this tradition that makes the selection of a data collection system crucial to a mapping firm, a decision which can easily influence the success of that firm. Planning
Because of the ability to store, manipulate, and share the digital data, planning the Hilliard-Rome project was more important than ever. Questions that needed to be considered included: How will layer names, color, line and point symbolization, as well as other attributes attached to the date make it easier to read and work with? What output scale or scales will the data be displayed at and in what coordinate system? What will the data be used for? (For example route planning, inventory, final design, or graphic data base-each would have a different level of accuracy and type of data included.) The System
Here, CADMAP digital mapping program from Carl Zeiss Inc. provided the ability to change any attribute in a point and click method or to batch-change multiple attributes of part or all of the file using a one page menu. Even changing the coordinate system itself requires only keying in the necessary transformation.
CADMAP was originally chosen seven years ago because it was developed with close ties to, and an understanding of, the needs of the photogrammetric mapper. The other strong point was that it offered tremendous flexibility in data translation. This was crucial because of the number of systems that were fighting to become the standard on the surveying and engineering desktop. CADMAP has evolved through the intervening years with powerful batch editing capabilities and customizing tools for the users which play a large part in the final edit process. It is also worthwhile to note that most mapping at M.A.N. is accomplished on Zeiss P series analytical stereo instruments. A CAD system cooperating fully with an analytical instrument via bi-directional drivers can reduce the time required for both data collection and edit, while increasing the accuracy of the data gathered.
From the operator's perspective, mapping with CAD is unchanged in that he still needs to move the measuring mark from point to point recording objects on the ground. His judgment and measuring ability are still the key to recording the data accurately.
Even here, though, CAD offers some advantages. First, the function keys give him a systematic method of progressing through different types of data making sure nothing is missed. Completeness is also aided in that the mapping with its colors and symbolization is on the screen within easy sight of the operator. In many cases (as with CADMAP and the Zeiss analytical stereo-plotters) the mapping can actually be transferred directly into the optics so the operator never has to look away from the eye pieces.
Next, with bi-directional drivers (meaning the analytical instrument controls the CAD system which in turn can control the instrument) snapping line ends on the screen moves the plotter to the X,Y,Z position of the point being snapped. This allows the mapper to make sure that the right lines are being matched and that their endpoint coordinates are exactly the same. The mapping can also be done with little regard for things like fences walks and drives running into buildings since this will be easily remedied in the final edit.
Finally, computers have made it possible to store the ground surface as a DTM. CADMAP aids in data collection for the DTM by driving the analytical instrument to points on a regular grid which has the size and density determined by the operator. The program also offers standard all the tools necessary to transfer the raw data to an external engineering package, or the add-on function of contour creation from the DTM without leaving the program.
DTMs require only that the lines and points required to describe the ground be stored. The software that analyzes this data will connect the data points with lines forming triangles. The software can then analyze any given triangle to compute the elevation for any coordinate position within the DTM. Providing the terrain as a DTM offers the end user numerous benefits.
With DTMs revolutionizing how the ground is stored and portrayed, project managers are seeing many advantages over the historic methods of string digitizing contours. With a good DTM you can generate accurate contours and a host of other products, but it is nearly impossible to create an accurate DTM from contour lines.
Back in CADMAP the final touches were added for the Hilliard-Rome project, such as batch clipping line over-runs, labeling contours, turning appropriate DTM points into visible spot elevations, adding text, title box, and coordinate grid . The final plot was then made using a variety of supported plotters at the desired scale for a single sheet, or up to 100 in batch mode.
The final step of translating data to the target system was probably the most important function of CAD. This step would provide countless users within the city the ability to share the same data, modify it to meet their needs and, if necessary, pass it along to the next phase. CADMAP offered us the flexibility to customize each file for the particular system it was being transferred to-having layer names, colors, and the graphic appearance of customized lines and symbols, if necessary-providing the end users with a product they can use as a base data set to accommodate all current and future growth. About the Author:
Dennis Shimer is a 1982 graduate of Ferris State College in Big Rapids, Mich. with a degree in Photogrammetric Mapping Technology. He has 10 years experience in the mapping industry. He has been with M.A.N. Mapping Services Inc. for the last eight years as a senior stereo plotter operator and manager of a UNIX based CADMAP system. He may be reached at 614-876-3663. Back |
