5.2. Acquiring the Data You Need

Once you have determined your mapping and data needs, you have to find the appropriate data. To find such data, you can, for example, search the Internet, trade with friends, or download from a government site. This chapter takes you through several examples from searching for premade maps to finding data to do your own mapping and conversion.

5.2.1. Finding Premade Maps

Sometimes all you want is a premade, basic map of the world or a specific country. The Internet can be a good place to find these. One excellent site is the CIA World Factbook at http://www.cia.gov/cia/publications/factbook/geos/ag.html. Here you will find a basic political map for every country and also a detailed demographic, cultural, and political summaries. Large-size, print-quality reference maps are also available in Portable Document Format (PDF).

Another good resource is the University of Texas Online Map Library at http://www.lib.utexas.edu/maps/. With a brief walk through some of the links, you can find diverse maps that show, for example, a street map of Baghdad or a shaded relief map of the United Kingdom, nicely showing the Scottish Highlands. The library has scanned many of these maps and made them available to the public domain.

5.2.2. Finding Satellite Images or Shaded Surface Maps

Premade maps from images or shaded elevation models are also available. The CIA Factbook (http://www.cia.gov/ciapublications/factbook) has small political maps for each country. Other reference maps that show more physical data (e.g., bathymetry and elevation) or even photographic examples of the earth can be found. The U.S. National Geophysical Data Center (NGDC) includes some beautiful premade gallery images of their more detailed source data; look for them at http://www.ngdc.noaa.gov/seg/topo/globegal.shtml.

5.2.3. Finding Map Data

The Internet contains a wealth of geographic information for your projects. It is a natural place to begin your quest for mapping data. Some government agencies make their mapping data freely available on the Internet. Certain organizations have even made it their mission to make mapping data available for others for free. Other groups have created catalogs of mapping data you can search.

Plenty of Internet map data sources are available for the United States. North American data is also widely accessible, with Canada having some excellent products. Global data is more of a challenge. International organizations such as the United Nations or even American-based organizations that observe global events, such as the National Oceanic and Atmospheric Administration (NOAA), are some of the better sources for global data.

There are a variety of data repositories on the Internet to choose from, and they each have a certain target scale of application that can't be ignored. For example, a satellite image can show pictures of where you live. Figure 5-6, for example, shows the different colors of surfaces such as roads, fields, and waterways, but only to a certain level of detail. If you hope to use it for planning a walk to the corner store, you will be sorely disappointed. However, if you want to see how your city is laid out, you will probably be satisfied. Figure 5-7 shows a closer look at the same image as Figure 5-6. The maximum amount of detail has been reached, and the image looks bad. This is an attempt to use the image beyond its intended scale of application.

There are multiple ways to hunt for good data. You can announce your search to email mailing lists or review web sites that summarize free data sources. Local mapping companies and government agencies are also a good place to start. Many states, provinces, and municipalities have their own GIS and mapping offices. Your search can also include local consulting companies, industries, or nonprofit organizations related to environmental or natural resource management and use. Just ask to speak to someone in the mapping or GIS department. Most people in these organizations

Figure 5-6. A Landsat satellite image showing the region around Toronto

Figure 5-7. A closer (though fuzzy) look at the Landsat image used in Figure 5-6

are willing to discuss what data they can share. If they can't share the data, they can at least point you to the original source for you to seek out on your own. Table 5-1 lists a few web sites and email lists that include free map data or people who can help you find data.

Table 5-1. A few web sites and mailing lists offering free map data resources and discussions

Web site	Description
Canada GeoBase Portal	http://www.geobase.ca/
Canadian Government	http://geogratis.cgdi.gc.ca/download/
Earth Observation Portal	http://maps.eoportal.org/
FreeGIS.org data/mailing list	http://freegis.org
GeoConnections Discovery Portal	http://geodiscover.cgdi.ca/
GIS Data Depot	http://data.geocomm.com/catalog/
Global Change Master Directory	http://gcmd.nasa.gov/
Global Land Cover	http://glcf.umiacs.umd.edu/data/
GTOPO Elevation Data	http://edcdaac.usgs.gov/gtopo30/gtopo30.asp
MapServer list	http://lists.gis.umn.edu/mailman/listinfo/mapserver-users
NASA Topography	http://www2.jpl.nasa.gov/srtm/dataprod.htm
U.S. Geologic Survey	http://edcsns17.cr.usgs.gov/EarthExplorer/
U.S. Maps and Data	http://geodata.gov
U.S. National Atlas	http://www.nationalatlas.gov/atlasftp.htm
U.S. National Geospatial Data Clearinghouse	http://clearinghouse1.fgdc.gov/

Best Data Not Always Available

Organizations and individuals who maintain data repositories are called custodians. Some custodians aren't interested in sharing their data with the public. This is a well debated topic, especially for regional and federal governments who have, traditionally, been the custodians of massive government map data repositories. Sometimes they are the only ones who use the data and are often responsible for keeping the data unavailable.

As digital mapping capabilities have infiltrated various domains, the pressure on governments to give data away (to companies, the public, nongovernment organizations and even other government departments) has increased dramatically. The arguments are usually somewhere between "Taxpayers paid for it, I have a right to use it!" and "Our department budget has gone down, and we need to recover costs, therefore you must pay for it." On the one hand, public agencies are funded by public tax dollars to collect the data. On the other, the same agencies may have tight budgets and will try to make money selling the data. So beware, the company or organization that has the best mapping data available may not be willing or able to share it with you.

There are some proactive ways to get access to data. If your project needs a certain type of data and you can help meet the objectives of the custodian as well, it might be a good idea to enter into a project together. One group puts time and effort into the project, and the other contributes data. A long-term data exchange agreement may also play a mutually beneficial role. The idea is that one party provides mapping data, and the other gets open access to it. In return, the user provides their updates of the data back to the custodian. Of course, each agreement should be negotiated to include only certain kinds of updates or sharing of only certain pieces of data (such as nonconfidential information).

This data sharing/update model is increasingly common, but at the same time many groups are making their information freely available for open use. It is important to get a broad feel for others who are also interested in, or may already have, the data you are looking for. Data can be shared without having to copy source data by using OGC web specifications. For example, the satellite image in Figure 5-6 is from a free web service and requested by a mapping application. See Chapter 12 for more about the OGC and web services.

Finding and getting access to data aren't the only problems. Some data may be unusable or inappropriate for the task at hand. This goes beyond matters of scale and resolution, referring more to accuracy and supplementary attributes. One example is a forestry initiative of the government of British Columbia (Canada) to map certain types of pest-infected trees. Traditionally this has been mapped using a fixed-wing aircraft carrying a reconnaissance mapper who spots the trees and then draws their location on a paper map. At the office, the paper maps are digitized onto a base map in the computer. While these maps are useful for general landscape-level analysis, you would not want to depend on the map to lead you to a particular tree in the middle of a dense forest. Wise use and understanding of the appropriate application of your data will help you have realistic expectations when starting your mapping project.

The Internet isn't the only source of mapping data. Chapter 9 discusses how to create some of your own data with map-editing tools or using coordinates taken from a GPS receiver.