Wikipedia:WikiProject Maps/US locations/Technical specifications

Geographic data

The maps used for this project are generated from the TIGER/Line shapefiles provided by the United States Census Bureau. Any errors, omissions or inaccuracies in the data provided by the Census may be reflected in the maps generated by this project.

The current set of maps are generated using the 2012 data [1].

The maps are projected using the Albers equal-area conic projection, a standard used by both the Census as well as the USGS, for consistency. The standard parallels for each county are derived from the latitudinal extents of the parent state (plus or minus 1/6) so that all counties in a state are using the same projection. This allows county maps within the same state to be joined together if necessary.

File specifications

The SVG files are of variable widths and heights depending upon the geographical extents of the featured county, and set to display the county, plus the greater of 10% the height/width or 100 pixels (at full nominal resolution) as padding. The exact scale of the maps is also variable depending upon the latitude of the county, but is generally between 170 and 180 meters per pixel (at full nominal resolution). The coordinates used in the maps are rounded to the nearest 1/10 pixel to maintain accuracy while avoiding very large file sizes.

The maps are drawn with the origin at the upper left corner of the parent state and then translated so that the subject county is centered in the visible part of the file. Some entities that appear in the viewport but continue off the sides of the map, such as adjacent counties, are not cut off but continue into the unrendered space. These entities will be visible in SVG viewers that do not limit the view to the height and width specified in the document.

The maps employ cascading style sheets to style elements based upon class and/or ID attributes for consistency, ease of editing and file size concerns. Some SVG editors may attempt to style elements individually using inline styles which may cause substantial increases in file size. If possible, inline styles should be avoided.

The maps contain the following layers (groups), from bottom to top :

• Plain gray background (this represents land outside of national borders, if necessary)
• Lower resolution coastline, where applicable (needed to show coastlines extending beyond national borders)
• Subject county and adjacent counties with appropriate fill colors
• Incorporated and unincorporated places
• Water features
• Roads
• County border overlay
• State border overlay
• Locator circles (initially transparent, used to locate geographically small places)