3.2 Topographic Maps for Design Work

a. General Information

Project Engineers should determine as early as possible whether or not digitized topographic maps are required for a project. If needed, a request should be forwarded to the District Photogrammetry Coordinator. Blanket service contracts have been executed to digitize existing topographic design maps into standard Caltrans CADD format. Contract Administration is handled by the District Photogrammetry Coordinator and the Engineering Service Center (ESC) Photogrammetry Section with responsibilities similar to present contract procedures for obtaining compilation of photogrammetric maps.

The ESC Photogrammetry Section has developed a document entitled "Drafting Standards and Symbols For Design Mapping By Photogrammetric Methods". The document outlines the standards to be used in the development of both hand drawn and electronically digitized maps.

In general, digitized topographic maps provide an accurate representation of a project. These maps are two-dimensional representations of surface topography and do not contain terrain information necessary to determine earthwork volumes. Although extremely accurate, the turnaround time from digitizing request to electronic maps can take up to 9 months.

A letter having the file specifications to be net copied or a tape of the digitized maps will be forwarded to the District CADD System Administrator for loading onto the system. Users will be notified when the digitized maps are available for design purposes.

b. Project Setup

1. Introduction

The Caltrans standards for metric design planes are 429 kilometer sides with subdivisions to the ten-thousandth (1/10,000) of a meter. For design work which is geographic in nature it is necessary to associate this design plane with real world coordinates.

Historically, two methods of associating the graphics plane with real world coordinates have been used. The first method defines the lower left corner, or global origin, of the square plane. The second method defines the center of the design plane. These two methods are not compatible and require the movement of the graphics data in order to correctly reference files to each other using coincident coordinate systems.

A system of planes is based on the California Coordinate System (CCS) and is designed to respect the various zones of CCS. Corresponding to each plane is a single coordinate value used as the "project setup" reference point. By applying this project setup coordinate value to the center of the design plane, all subsequent graphics can be placed and referenced using real world coordinates.

The purpose of the plane system is to provide uniformity of global origins for geographically referenced drawings in the same general area. This system will enable the user to easily attach as reference files all available geographically referenced drawings which have been established on the plane on which he/she is working. This system will greatly reduce the need to physically move, in order to properly reference, files in the design plane with different global origins but in the same area.

Early in a project the appropriate design plane should be selected by the project engineer. This allows for geographic consistency in establishing base maps, determining roadway geometrics and assembling the project for PS & E. Once the project plane is selected, this information should be available to the digitizing contractor (if used), the project development staff and the delineation personnel.

The importance of establishing this project setup coordinate cannot be stressed enough. Consistent use of this value will guarantee that files moving between the functional units will share a common coordinate system throughout the CADD process.

2. Design Plane Center Coordinates

Following section 3.5.b, is an overall map of the coordinate planes in California. Also following are listings of the established 9 metric design plane center coordinates and global origins for Metric.

3. Use For New Files

Once the correct coordinate system and center coordinates are determined, the "Project Setup" command can be found on the IGrds menu. By following the prompts the correct coordinate systems will be established. The menu command will also provide the capability to place and label the project setup symbol with the center coordinates on level 1.

All current photogrammetric mapping and digitizing projects will be referenced to the planes. If an old project is to be referenced to a new project, the old project will have to be physically moved to the correct location on the design plane after the global origin is changed to the correct plane values (see Use for Existing Files, below).

4. Use for Existing Files

If data has been placed on the design plane without using the Project Setup command, the coordinate value of the existing data will not be compatible with files created using the Project Setup command. Additionally, data placed using the IGrds program will not be compatible to non project setup files since the roadway design software is dependent upon project setup to establish the reference location and coordinate system for the placement of graphics data.

The following steps outline the procedure to reestablish a coordinate system on the design plane which is based on a project setup point:

5. Metric Mapping Conversions For Photogrammetric Mapping

(Old 1"=20')
(Old 1"=50')
1:1000 (Old 1"=100')
Grid Tick Size20 mm4 m20 mm10 m20 mm20 m
Grid Tick Spacing250 mm50 m200 mm100 m200 mm200 m
Text Height3 mm0.6 m3 mm1.5 m3 mm3 m
Contour Interval0.25m0.5m1m
Index Contour Interval1m2m5m
Spot Elevations0.01m0.01m0.01m
Cell Scale Factor0.1.0250.5

(Nearest Reading)

MicroStation Working Units - 429,496 squared meters
1000 millimeters per meter
10 positional units per sub unit

For conversion of existing contour maps from english to metric, show spot elevations and contour annotation to the nearest 0.01 m.

Use the U.S. Survey Foot to convert from english to metric.

39.37/12= 3.280833333 feet per meter

c. Scanned Maps

In addition to digitized mapping, the ability to scan data from hardcopy maps into an electronic format exists. This method has the advantage of a very quick turnaround time from request to electronic map.

The ESC- OE Document Scanning Unit has a 200 - 500 DPI (dots per inch) scanner that is used to scan Topo maps as well as other documents. If the project engineer determines that scanning is appropriate for a project, existing Topo maps may be sent to Headquarters for scanning. Include the following information: NAME, PHONE NUMBER, CHARGE NUMBER, whether the text is converted to ASCII text (the computer stores as text), or vectorized (lower quality, the characters are composed of lines) and the date the completed maps are needed.

The scanner is very similar to an electrostatic copier. It's function is to capture the image and route it to the workstation for processing. Documents can be on either paper or translucent drafting film. High contrast, original images give the best results. Maximum document size is 40" in width and 72" in length.

The scanner operator can manipulate the scanned image to add, move or delete information and also manually convert text to ASCII code. Conversion of the raster image to vectors is done automatically by the system. It takes about 30 to 40 minutes to convert a maximum size Topo map of rolling terrain. After the vector file is completed, it is converted to a dxf file and usually imported in MicroStation as dgn file. Users can then copy the file into a district directory.

There are some types of maps that make very poor candidates for scanning. These include; screened maps, maps with very light lines, maps with the text running into the lines, uneven maps (light and dark areas on the same map), folded maps (send paper maps in sturdy mailing tubes) and maps that have been taped together.