Values for antenna height above average terrain (commonly abbreviated HAAT) may be calculated using the form below. Two terrain databases are available here: the FCC's terrain database which covers the continental Unites States (48 contiguous states) and which has been used by the FCC since the early 1980s, and the GLOBE 1 km Base Elevation database from the National Geophysical Data Center. HAAT values may be calculated for any location on Earth, given coordinates and an antenna radiation center height above mean sea level (usually correllating to the physical center of the antenna). More about HAAT below...

Antenna height above average terrain, along with the Commission's FM and TV propagation curves is commonly used in the prediction of coverage by television stations and by FM radio stations (see Section 73.313 for FM radio stations and Section 73.684 for television stations) as well as some wireless radio services (see rule section 90.205). FM and TV coverage may be extended or shortened in a given direction depending on the terrain in that direction. For example, a mountain may reduce service (as compared to flat terrain), while a valley may allow expanded coverage. The HAAT value represents an average of the terrain elevations within 16 km (10 miles) of the transmitter site, and so provides a single value on which general coverage calculations and regulatory requirements (such as station classes) may be based.

How is the HAAT determined?   A HAAT value is determined by taking 50 evenly spaced elevation points (above mean sea level [AMSL]) along at least 8 evenly spaced radials from the transmitter site (starting at 0 degrees [True North]). The 50 evenly spaced points are sampled in the segment between 3 to 16 km (formerly 2 to 10 miles) along each radial. The elevation points along each radial are averaged, then the radial averages are averaged to provide the final HAAT value. Terrain variations within 3 km (2 miles) of the transmitter site usually do not have a great impact on station coverage.

Only the 3 to 16 km segment of each radial is used. When the concept of antenna height above average terrain was developed in the late 1930s and 1940s for VHF and UHF radio and television, the process for determining HAAT was a manual one. An engineer would lay out topographical maps covering a radius of 10 miles from the transmitter site, draw 8 radials spaced every 45 degrees, and then find the elevations for 50 evenly spaced points out to 10 miles from the transmitter site for each radial, and then average the points for each radial between 2 and 10 miles from the transmitter site. Needless to say, this procedure was very tedious. In 1984, the Commission formally permitted applicants to use terrain databases in the computation of HAAT (see the July 13, 1984 Public Notice), and permitted applicants to use additional evenly spaced radials. However, the Commission did not extend the radial segment to be considered beyond 16 km.

Terrain Roughness adjustment cannot be used. In 1975, the Commission adopted a terrain roughness adjustment for predicting coverage, and shortly thereafter suspended it after numerous reports of inconsistent results. That rule suspension remains in effect today. See the Development of FM and TV Propagation Curves documents posted on the FCC's website.

HAAT values here may not match the values listed in the FM Query or the TV Query. Several factors may influence the determination of a HAAT value. Different terrain databases, or manual computations using topographical maps, may produce different interpolated elevations for points along a given radial. The number of evenly spaced points sampled along each 3 to 16 km radial segment also influences the average elevation AMSL determined for a given radial. Varying the number of evenly spaced radials will change the final HAAT value. For this reason, the Commission has retained the manual computation method as the standard in cases where the HAAT is of questioned accuracy. In most instances, HAAT values determined here will be suitable for applications submitted to the Commission, although use of this program is not required. For clarity, applications to the FCC should state which terrain database was used and the number of evenly spaced radials considered.

Notes about this HAAT program. This HAAT program uses the full 3 to 16 km radial segment, and will not calculate radial average elevations AMSL for truncated radials (shorter than 16 km). Users in the continental USA may select either the FCC's database or the GLOBE data. Where the FCC's terrain data is not found (for all areas outside the continental USA), the program defaults to GLOBE terrain data. Users in the USA may enter coordinates in either the NAD27 or NAD83/WGS84 coordinate systems, and this program will make the appropriate conversion. Users outside the USA should use NAD83 or WGS84 coordinates, although in most instances any difference in the HAAT values is likely to be small.

Source code for this program is not immediately available. This HAAT program is based on code used by the FCC to read the agency's 30 second terrain database, as well as additional Fortran posted by Mr. Greg Hand (now retired) at http://elbert.its.bldrdoc.gov/globe.html.

Questions or comments may be directed to Dale Bickel, dale.bickel@fcc.gov.