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Nevada DOT Steps Up Photogrammetric Work
The use of analytical stereoplotters combined with resourceful hardware and software upgrades provides CAD/GIS solutions for Nevada's state department of transportation.
By Howard Earl Analytical photogrammetric mapping is the most efficient way to compile accurate vector-based maps. Until the late 1970s such mapping was most often performed using analog (mechanical) stereoplotters. To meet the increasing demand for CAD and GIS data, analytical stereoplotters must make a quantum leap forward with regard to efficiency and simplicity when compared to the technology of the 1970s and 1980s. Today, notwithstanding the advent of competitive photogrammetric softcopy systems, the analytical stereoplotter is still the number one workhorse for planimetric mapping data. In Carson City, Nev., the state department of transportation (NDOT) has found that the proven technology of analytical stereoplotters coupled with resourceful upgrades of hardware and software provides an excellent solution for the generation of CAD and GIS data. NDOT complements production at half the cost
The process through which a transportation system is constructed in Nevada is similar to the way it's done in any other part of the United States. First, a need for the construction of an expanse of highway must be established through safety or economic criteria. State and/or federal funding may be appropriated and NDOT's Design and Location Divisions conduct their respective feasibility studies. After route corridor selection is established, a photogrammetric survey may be conducted. The compilation of photogrammetric data (including planimetric and topographic data in the form of feature coding and DTM/ttn - triangulated tin network - files) at the NDOT supports CAD functions for all divisions of the department.
One of the most critical aspects of the DTM data sets is their use to estimate preliminary pay quantities (volumetric earth). These estimates are used initially by the private contractors to estimate the cost of construction for bid purposes. After the bid is won and the construction is complete, the NDOT Location Division/Photogrammetry section may be called upon to generate an "as built" volumetric data set (final ground) to be used as arbitration data in the event of pay quantity disputes between NDOT and the contractor.
Carol DeMar, photogrammetric production manager at NDOT, recently compiled photogrammetric CAD data involving the reconstruction of a multi-million dollar interchange project located in North Las Vegas at Lamb Blvd. and US Interstate-15.
The first step in the Lamb Blvd. project was to have a field survey performed. The surveying department at the NDOT went into the field and identified 16 points to be used as control on the project. The coordinates for these control points were derived using GPS methods. Next, 16 vertical photos of the project area were taken at a flying height of 1500' covering strips a total of two miles in length. The film was processed into prints and diapositives for photogrammetric work.
To facilitate their photogrammetric work, NDOT recently acquired the DMA (DAT/EM Mapping Analytic) stereoplotter. This is a newly developed upgrade of Intergraph's IMA (Intermap Analytic) stereoplotter. The IMA (based on the Zeiss P1) has always been an excellent instrument with an overall accuracy of ±2 microns (rmse - root mean square error). In 1991, Intergraph stopped producing the IMA because its MicroVAX and resident mapping software had become obsolete. Recognizing the investment that companies have in this instrument, DAT/EM developed an upgrade whereby the MicroVAX is replaced with electronics which drive the stage plates and zoom controls. A CAD PC is interfaced with the system so that the mapper may compile data directly within AutoCAD or MicroStation.
The DMA will be NDOT's primary stereoplotter used to compile data needed for arbitration disputes until softcopy establishes a track record for such projects.
All aerotriangulation was performed using Erio Technologies' Albany block adjustment software. This software performs a Least-Squares adjustment on the control point coordinates so that any error is spread throughout all the models that comprise the project.
Finally, the standard interior, relative, and absolute orientations are performed with the first two diapositives placed on the stages. During the orientations the DMA's 10-40X zooms often come in handy allowing the operator to zoom in and verify that the floating mark is exactly over the fiducial.
Now planimetric feature mapping may begin. Lines are drawn around buildings, roads and parking lots. Numerous symbols are placed signifying light posts, fire hydrants, trees etc. The digital file is injected back into the optics of the DMA so that it is superimposed on top of the photographic imagery. This allows the operator to recognize errors (e.g., an improperly outlined building for example) and rectify omissions (e.g., a light post with no symbol).
Once a model is complete, the DMA's base-in/base-out functions make switching models easy. Only one diapositive must be moved to change models. For example, imagine you have just finished working on a model comprised of diapositives 6 (on the left stage plate) and 7 (on the right stage plate) and wish to move on to the next model made up of photos 7 and 8. It is not necessary to move photo 7 to the left stage and insert photo 8 onto the right stage. The operator must only insert photo 8 onto the left stage and put the DMA into 'pseudo' mode telling it that the placement of the diapositives is the reverse of the convention set up for that project.
The DMA was used to capture data that meets 1 foot contour accuracy specifications for surfacing (TOPO/ttn) and feature coding (planimetric/mfc - map feature coding) computations as requested by the department's Design and Right-of-Way Divisions. The operator observed points which when compared to field ground truth points measured with Leica bar code levels showed a 2 cm rmse in height.
A key feature of this mapping solution is the ease with which the compiled data can be shared with softcopy stereoplotters currently used in the photogrammetric profession (such as DAT/EM's DIGITUS and Leica-Helava's DPW 770). Through a concerted effort by Leica-Helava, NDOT, and DAT/EM, the DPW 770 softcopy systems at NDOT will soon output automated aerotriangulation (HATS) results to the DMA. This means that there is no time wasted on a pugger or comparator. Rusty Autry, chief photogrammetrist at NDOT, explains, "Because NDOT's softcopy, analytical, and semi-analytical systems all compile data using DGN/CAPTURE within the MicroStation environment, all files can be shared except the digital orthophoto and multi media products compiled by the softcopy systems. The value of this sharing of photogrammetric data should be self evident to any practical photogrammetrist restricted by limited staff, deadlines and cost in the changing world of technology."
In choosing the DMA to satisfy Nevada DOT's planimetric mapping needs, many factors came into play. One consideration was that the DOT already had an IMA which represented substantial financial investment. Additionally, their users were familiar with the operation of the instrument, so minimal training was required. Other benefits which the DMA offered over the original IMA were more efficient setting of models, less down time (due to relaxed environmental restrictions) and faster, easier to use, more efficient mapping software.
The next assignment scheduled for the DMA is a 1.8 mile mapping project with the same accuracy specifications and data sets as the I-15 and Lamb Blvd. project. The Geiger Grade project, as it is known, comprises part of the highway which connects Virginia City with the new $58,000,000 Freeway Extension (US 395) south of Reno. About the Author:
Howard Earl has a B.S. degree in Electrical Engineering from the University of Alaska, Fairbanks and over 5 years experience in photogrammetry and mapping. He may be reached at 907-274-3681.
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