3. Procedural Guidelines
3.1 Process For Project Plan Preparation With CADD
- The Caltrans CADD process is a complex flow of electronic information through a variety of functional areas. The process flows back and forth from mapping applications to engineering design and final project plan sheets.
a. Control Surveys
b. Obtaining Electronic Maps
- These technologies are used extensively throughout the project development process. The Global Positioning System (GPS) is a method which uses data obtained from orbiting satellites to establish basic horizontal control. Total Stations and Electronic Data Collection devices are more conventional ground survey methods.
- Once the requirements and funding for a project have been established it is necessary to obtain electronic maps which detail existing features and topography. These maps may be generated from aerial photographs, from scanned or digitized hard copies of maps, or from field survey data. The method chosen is dictated by the budget and time constraints of the individual project.
1. Electronic Maps from Aerial Photographs
- To obtain mapping from aerial photography, the project engineer first consults with the District Photogrammetry Coordinator (DPC). The Engineering Service Center (ESC) Photogrammetry Section together with the local district survey staff and DPC, will define the requirements for the stereo photography and establish premarks on the ground to be used for subsequent geometric control. The photography can be used to produce direct digital mapping, digital terrain models, and photogrammetric cross sections.
- Direct digital maps are compiled in 3-D directly from the aerial photography into a CADD file by a photogrammetric plotter operator, edited on an Engineering Workstation ( EWS), and plotted as a final product. The deliverables are mylar final plots and the CADD files. The 3
D features include contours, spot elevations, roadway delineation, waterways, retaining walls, and terrain break lines. These features are used as input to the CAiCE digital terrain modeling system.
- Using direct digital mapping, a geometrically correct three dimensional file is created with features from the aerial photographs resolved into vector graphics. This is a MicroStation file and conforms to Caltrans CADD standards.
- The districts maintain a topographic library on their local computer network where the information can be shared by designers from various squads and functional areas for project development.
- The advantages and disadvantages that exist with these maps have come to light at Caltrans. First, from initial request to actual availability of the electronic map file, a lag from 6 to 9 months can be expected. Though,the maps produced as described above are advantageous because they are extremely accurate and detailed, and in many cases provide current information which cannot be reflected on more conventional maps such as those provided by the USGS.
2. Scanned Maps
- The second method of obtaining electronic maps is through scanning. This process will allow an image on paper or microfilm to be 'captured' and placed in an electronic file. The process used at Caltrans is for the project engineer to obtain an original document. The engineer can indicate removal of extraneous information from the original, add geographic control points and perhaps preliminary design. The document is sent to the ESC Office Engineer Document Scanning Unit for scanning. When complete, the Document Scanning Unit will notify the district to copy the electronic map from the Headquarters computer directory to a local computer.
- Scanning has the advantage of being able to quickly generate electronic maps, with turnaround time from request to delivery generally less than one month. Also, the costs are small compared with obtaining maps from aerial photographs.
- Several disadvantages exist with generating electronic mapping by scanning. The scanned images are captured from the original document into the electronic file at a scale of 1:1 so that any scanned data must be scaled up to become useful in design. All information is placed in the electronic file on a single level and requires subsequent editing to conform to CADD standards. Text, shapes and closely spaced parallel lines are not resolved exactly, and additional editing on the DWS is required.
3. Digitized Maps
- The third method of generating electronic maps is by digitizing aerial photographs, existing maps or existing plans at a DWS equipped with a digitizing table. Some of the drafting workstations purchased by Caltrans are equipped with a 42" x 60" digitizing surface. The original map can be fixed to the surface, control and scale established, and the data digitized or traced into the electronic file. This is done by DWS operators in the district offices and by private digitizing firms under contract to the district.
- This method has the disadvantage of being time consuming and requires some skill on the part of the workstation operator. The advantage is that the design squad has the ability to generate a map containing only the pertinent information. Furthermore, the turn around time is limited only by the availability of the operator and the workstation. The compliance to CADD standards and geometric accuracy using this method is generally good.
4. Electronic Maps from Ground Surveys
- The fourth method of obtaining electronic maps is through ground surveys or field data and is collected by district survey crews. A survey request is made by the project engineer to the district survey staff. A field crew obtains the data using Total Station equipment with electronic data collection capabilities. This data may be reduced into a terrain model data base.
- In addition to electronic maps, field collected terrain models provide the base data necessary to generate design earthwork volumes using the various roadway design packages.
- Critical points from the data file can be plotted into a graphics file and transformed to map information. Contours can be plotted as well as critical grade breaks in plan or profile. This method has the advantage of speed in acquiring data. In addition, there is a direct correlation between electronic map data and field notes and maps can easily be supplemented by more field data. Accuracy is good and the workstation operator has control over the levels on which data can be placed. All map data in the electronic file is geometrically correct.
- Once the topographic maps have been established, these graphics files are available for use on the EWS. For preliminary design, Caltrans engineers use IGrds software to define, modify, and display horizontal and vertical alignments. The preliminary designs are transferred to various functional area groups such as Right-of-Way Engineering and Traffic Engineering. The functional areas begin their design phases based on this preliminary design. The geometrics generated on the EWS are also available to delineators on a DWS for assembly of the plan sheets.
6. Plans, Specifications and Estimates (PS & E)
- The design process continues until the geometrics and Right-of-Way are established and approved within the district office. The engineers continue to use IGrds software on the EWS to finalize design cross sections and earthwork volumes while the functional areas such as Traffic, Hydraulics and Landscape finalize their project plans.
- The plansheet files are assembled by the delineator on the DWS using information provided by various functional areas. The Project Engineer's electronic signature is affixed to the finished sheets. The group of files is then transmitted electronically to the ESC Office of Office Engineer (OE) for review. Changes deemed necessary to the electronic plans are discussed with the district project engineer and are made by ESC- OE Drafting Services. The nature of CADD allows these changes to be made quickly.
- After final changes to the plans are made, reduced plans are generated for the competitive bid process. The final electronic plans are transferred back to the originating district and archived until completion of construction. Once construction is complete, as
built changes are recorded on the electronic files. These files are then plotted and sent to headquarters for micro filming. The electronic files may be archived in the district and recalled for future projects when necessary.