Presented by Colorado Bird Atlas Partnership:

The Second Colorado Breeding Bird Atlas


Atlas Methods ~ Blocks & Regions

priority block count

Atlas Blocks

Atlas II collected data throughout Colorado, using U.S. Geological Survey 7.5-minute topographic quadrangle maps (quads). Dividing each quad in half vertically and in thirds horizontally results in six equal blocks (Figure 1). Uniform selection picked the southeast (SE) portion of

each quad as the “priority block,” the primary block for data collection. Comparing counts of priority blocks between Atlases provides a gross measure of “change.”

In addition to priority block data, atlasers collected data in some non-priority blocks during both Atlases; however, the amount and distribution of non-priority block data did not meet the same standards as priority blocks. Both Atlases incorporated both priority and non-priority block data into the distribution and change maps; however, block statistics tally priority block data only. The maps show distribution by quads and incorporated observations from both priority and non-priority blocks.

Block Codes

Atlas II defined block codes based on an abbreviated code system for U.S. Geological Survey 7.5-minute topographic quadrangle maps (quadrangles) and the position of

blocks within quadrangles. Block codes contain 6-digits.

  • The first two numeric digits represent the latitude and longitude of the quadrangle, with only the last number of the latitude and longitude represented.
  • The middle two alpha-numeric digits define the location of the quadrangle within a latilong. A latilong consists of eight rows of eight quadrangles within a latitude/longitude “block”. Within latilongs, the letters A–H correspond to the location of quadrangles from north to south, and the numbers 1–8 correspond to the position of quadrangles from east to west. The latilong numbering system starts from the southeast corner of the latilong.
  • The last two digits of the block code represent the location of the block within the quadrangle, i.e., SE, SW, CE, CW, NE, and NW.

For example, the Alamosa East-SE block, with a block code of 75D7SE, occurs at 39° north latitude and 106° west longitude. Within the 39106 latilong, ‘D7’ places the quadrangle as the fourth map north and the seventh map west from the southeast corner. The “SE” places the block in the southeast portion of the quadrangle.

Atlas Regions

For administrative purposes, Atlas II divided Colorado into 26 Atlas regions with a varying number of blocks per region. Each region

had a Regional Coordinator assigned to recruit volunteers, manage block assignments, and review data records. Click on a region below for a more detailed view.

All Regions Map