ETL 1110-1-184
01 Oct 1998
accuracy between a CORS site (NAD83-1996) and a NAD83(1986) site
is usually better than 0.5 meters, but can be up to 1 meter.
c. ITRF. The ITRF position listed at each CORS site is
ITRF94 and is based on the epoch date of 1996. ITRF is used
mostly for studying plate tectonics and applications related to
crustal motion. Unless the project will be referenced to ITRF,
do not use these coordinates.
d. Other Datums. For high precision (centimeter-level)
surveys, a key issue is to have consistent project control. If
there is existing project control, hold these values fixed in the
final adjustment, rather than the CORS. Once the positions are
in NAD83 or NAD83 HARNs, conversion programs such as CORPSCON
(can be downloaded from http://www.tec.army.mil) can be used to
convert the final positions from the data processing. Note some
conversion programs can introduce errors up to one foot when
converting from NAD83 to North American Datum of 1927 (NAD27) in
certain geographical areas.
e. Vertical. The vertical component of a CORS site is
based on the ellipsoid height. For NAD83-1996 positions, the
ellipsoid height listed is based on the GRS80 ellipsoid. For the
ITRF positions, the ellipsoid height is based on the ITRF
ellipsoid. These ellipsoid heights should not be confused with
orthometric heights obtained from spirit leveling. Geoid
modeling can be used to convert the ellipsoid heights with the
NAD83-1996 positions to North American Vertical Datum of 1988
(NAVD88).
Data Formats and Sample Rates.
B-3.
a. Data Format. The data provided from the CORS sites are
in Receiver Independent Exchange version 2 (RINEX 2) format.
Most, if not all, post-processing software packages accept data
in this format. The data for a CORS site is stored in either
hourly or daily files, depending on the site setup. The hourly
and daily data files are based on UTC time. Both types of files
are in a compressed format and need to be decompressed by the
GZIP or GZIP386 program. Other ZIP programs might not decompress
the files properly. The GZIP386 program can be downloaded from
the Software Menu on the CORS homepage.
b. File Naming Convention. The RINEX file naming convention
is as follows: {SSSS}{DDD}{H}.{YY}{T}; where SSSS is the four
character site identifier, DDD is the day of year, H is a letter
which corresponds to an hour long UTC time block, YY is the year
and T is the file type. For daily files, the format would be
B-2
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