TIGER Map Service The "Coast to Coast" Digital Map Database.

Tiger Map Service
Frequently Asked Questions (FAQ)

Q: Are there any legal/copyright issues using these maps?
A: No. The maps are public domain. You may use them how you wish.

Q: How can I download the images I create?
A: That depends on your browser... With NetScape and Microsoft Internet Explorer you can Ctrl-Right Click *on* the image and you will get a menu with an option to "Save Image As...", or "Save Picture As...". Then you can just give it mapname.gif.

Q: The map image is broken. What gives?
A: This could be one of a number of problems, but the most frequent is a browser problem... Some browsers don't recognize the HTML code <INPUT TYPE=IMAGE...>. I think Netcom NetCruiser still has this problem, and older versions of other web browsers. You'll need to upgrade your web browser.

Q: Why is this server sooo slow?
A: These machines generate 100,000 to 375,000 maps per day. The code is not terribly efficient and at times there are a *lot* of users hammering on the machines. All I can say is be patient.

Q: Where are the street names?
A: We do not have funding and resources to improve/enhance the map server. There are some programming problems with font rotation and scaling which need to be solved. If you have any experience in this area, please send me some e-mail.

Q: Sometimes forcing on features doesn't work. What is the problem?
A: There are hard coded thresholds for feature layers. Most of the features won't display until you get to a map scale around 1:1500000. This threshold was put in place to keep users from swamping the CPU with unreasonable requests. (e.g. draw all streets at the U.S. Level)

Q: The legend on the Census Statistical Maps display garbage occasionally. What's up?
A: This is a known bug that we encountered when we moved the code to the SGI machines. I am aware of it and will fix it, hopefully in the near future.

Q: What happened to the resize map option? (i.e. pixel height and width options)
A: It is still available from the URL http://tiger.census.gov/cgi-bin/mapbrowser. When we added the new border graphics around the map, I couldn't figure out how to resize them on the fly while preserving the integrity of the border image. When I tried it, it became distorted.

Q: Which browser do you recommend to best view TMS maps?
A: I test with the latest versions of both Microsoft Internet Explorer and NetScape Navigator. Everything looks and runs good with these packages. If you are running an old browser, try the old mapper at URL http://tiger.census.gov/cgi-bin/mapbrowse. We won't be redesigning anything to accommodate old browsers and we don't particularly want to hear about the problems with them. Keeping your browser software current is *highly* recommended.

Q: Why is this "geographic entity" not at the correct location?
A: The Census geographic product (Tiger, GICS, STF) definition for geographic entities is based on an "Internal Point" which has a technical definition as follows:

INTERNAL POINT

An internal point is a set of geographic coordinates (latitude and longitude) that is located within a specified geographic entity. A single point is identified for each entity; for many entities, this point represents the approximate geographic center of that entity. If the shape of the entity caused this point to be located outside the boundaries of the entity, it is relocated from the center so that it is within the entity. If the internal point for a block falls in a water area, it is relocated to a land area within the block. On machine-readable products, internal points are shown to six decimal places; the decimal point is implied.

All of the geographic location (lat/lon) values from the "New Location" button and the U.S. Gazetteer are derived from these calculated values. Alternate geographic values may be obtained from the USGS Geographic Names Information System (GNIS) database or by telnet at the site martini.eecs.umich.edu at port 3000. Please remember the TMS input is in decimal degrees and not Degrees:Minutes:Seconds.

Q: When I enter the lat/lon coordinates of my house from my GPS receiver, the mapped position is wrong.
A: There are two sources of error. First, the U.S. Department of Defense intentionally "dithers" the GPS for civilian users, supposedly for national security reasons. The errors can be as great as 100 meters, though they can be reduced by averaging positions over time. Second, the GPS and the maps here use different geodetic "datums". GPS works in the "World Geodetic System 1984" (WGS-84) while our maps use the "North American Datum 1927" (NAD-27). The difference between these datums can be as much as 100-200 meters within the continental U.S. Most GPS units can be programmed to display lat/long results in NAD-27.

Q: How can I add marker(s) to the output image?
How can I get the LAT/LON of a specified point?

A:

• Adding a Single Mark or Obtaining LAT/LON for a specified point:
  For a single marker, you can choose "Place a Marker" on the radio buttons to the right of the map section that says "CLICK IMAGE TO:" Now, look down below the map, under "Place Marker on Map" and select the desired "Symbol". Then click on the map itself. It should redraw with the marker placed where you clicked. Once the map redraws, scroll down to "Place Marker on Map" section and you will see the LAT/LON of where the marker was placed. (That is how you get the LAT/LON of a place you clicked)

• Adding Multiple Marks on the URL:
  For an example of placing multiple markers on the URL, the syntax is: mark=lon,lat,symbol,label;... an example:
  http://tiger.census.gov/cgi-bin/mapsurfer?lon=-77.028&lat=38.89&wid=.06&ht=.01&mark=-77.0364,38.8973,redpin,White+House;-77.01,38.8895,blueball,Capitol

  Here is an inventory of available symbols. If you would like a custom symbol, please send e-mail to mapsurf@geo.census.gov.
  Multiple Marks may be placed on the URL only. There currently is not an interactive mechanism for placing multiple markers on the map, one at a time.

• Adding Multiple Marks from a remote file/URL (MURL):
  For *many* marks, you will need to review the URL Look at http://tiger.census.gov/instruct.html and look at the &murl option. Deriving marks from a remote file.