2D Maps - Overlaying Digital Line Graph (DLG) Reference Layers

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1. Using Geospatial Viewers to Create Base Maps
2. Creating Base Maps with Geospatial Mapping Programs

DOP background	DEM background	Digital Line Graph (DLG)

Using Viewers to Create Base Maps

Spatial viewers like Global Mapper and ArcExplorer make the combination of aerial photo and geo-referenced DLG vector layers easy. In Global Mapper, for example, you'd use "File/Open into Current" or click the "Open into Current" icon in the toolbar as shown below on the first image to make layered base maps. Use the file dialog to find each layer you want to add. Global Mapper was used to make the three layered views shown above. (The free version of Global Mapper is limited to opening four layers. The only limit for the registered version is the memory of your PC.)

Professional mapping software companies offer free viewers that can open their native file formats. ESRI's ArcExplorer, for example, is useful for making base maps if you have access to data in ESRI format. The procedure to add layers in ArcExplorer is similar to Global Mapper, except the ESRI program calls the command "Add Theme".     In ArcExplorer, all the opened layers must have the same projection. If not, they won't line up. One of the advantages of Global Mapper is that data with different file formats and different projections can be mixed. The program automatically changes incoming data to match the open layers. Global Mapper can also work with compressed archive files …no need to unzip "SDTS.tar" files first. (If you do unzip SDTS archives, be sure to place each in its own subdirectory folder. If you don't, similarly named internal files will end up overwriting one another.)

If you create image/line base maps in a viewer, you will need to export the screen to use either in a drawing program or geospatial mapping program if you intend to add information. (Viewers are not generally designed to allow you to create new information or to add annotations to the view. One exception is the free ERDAS MapSheets Express. With it, you can add text, lines and polygons to a view and save the resulting image. If you exit the program, however, there is no way to save the work space for future modifications.) Saving images from viewers to use as base maps can be accomplished through screen capture programs or image export commands. If you use the registered version of Global Mapper, it can save the screen image complete with geo-referencing coordinates.

If you go to sites like GeoCommunity, USGS Earth Resources Observation Systems (EROS) Data Center, the UN GeoData Portal or the US Fish and Wildlife Service, you'll find many DLG vector data layers available for download. (See Basic Map Data and Links for additional data sites.) As an example, some of the files that are shown above in the Global Mapper "Open" dialog box refer to the following data types (all from GeoCommunity downloads):

When you visit sites with US data for download, the file folders are often identified with either the 100K or the 24K USGS quadrangle names. You can find quadrangle index names at the following sites:

• USGS Quad Index (downloadable files to view in Global Mapper)
• Quad Indexes at GeoCommunity

Using Geospatial Mapping Programs to Create Base Maps

Professional mapping programs are designed to read spatial data and add user created areas, lines and other data. Map Maker Gratis is one of the best free programs and an excellent tool for making basic maps. It loads tiff, jpg and bmp image files; ESRI shp and AutoCAD dxf vector files; and native Map Maker image and vector files. If the data originate from scans or other un-referenced sources, Map Maker has a simple procedure to establish the image scale. Geo-referenced data can be added as layers, which Map Maker will correctly scale based on the corresponding geo-referencing files.

Above is a Map Maker base map showing an aerial photo downloaded from TerraServer and a DLG layer. The vector data is for section lines from a free public land survey file in SDTS format and converted with Global Mapper to an ESRI shape file.

Note that in Map Maker you can change the scale by double-clicking the scale bar. In this case, 1:15,840 will accurate print at four inches to the mile. A scale 1:7,920 would print at eight inches per mile.

To open layers in Map Maker, right-click in a new, empty screen and choose "Project Manager" from the dialog box that opens. Use the "Add layer" button as shown below to locate information you wish to work with.

Map Maker will allow you to select from the file types shown in the following drop-down list:

Map Maker Gratis and Global Mapper together formulate a flexible mapping system at little expense. One way to make Map Maker Gratis more powerful is to first prepare data in the registered version of Global Mapper. You can then utilize image and line data from many sources and convert files with Global Mapper to formats recognized by Map Maker. Global Mapper can also prepare the data by changing various projections and datums to a uniform type that will work together in Map Maker.

If you like Map Maker Gratis, you might want to check out Map Maker Pro. The developer offers a free 30-day trial of the full-feature professional version of the program.

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