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ANSCIA Barrow GCP Database v1 MetadataDetails about collection procedures, processing steps, precision, contact and citation information, etc. Specific Notes:GCP_ID: A unique code for each GCP in this database. Note that a few GCP codes (C004, C033, C057, C058) are not listed, as the GCP was aborted in the field, or the feature could not be confidently identified on aerial photographs. Latitude and Longitude (DD): Decimal degrees, horizontal datum is WGS84. UTM Northing and Easting (z4, m): Zone 4 North, horizontal datum is WGS84, calculated from geographic coordinates using ArcInfo 8.1. Elevation (m): Vertical datum is WGS84. See DGPS Processing note below. Where antenna height was measured on a tripod or feature, elevation refers to that of the nearby ground surface. Where antenna height was not measured, the stated elevation refers to that of the antenna, as noted in the elevation and comments fields. For example, with GCP C017, the antenna was placed on the corner of a building's roof, and a note was appended to the elevation field: "13.73 m (on roof)". Horizontal and Vertical Precision (m): Average horizontal precision is 1.6 cm, and average vertical precision is 3.5 cm. These values do not incorporate antenna setup errors. See GPS Processing note below. Air Photos: With two exceptions, the vertical close-up and overview images (North is upper left) were created by cropping and editing scanned images of year 2000 proprietary aerial photographs from AeroMap U.S. The information listed is the year and frame number of the photographs that encompass the GCP feature. In two cases, the aerial photography used for the vertical images is from the USGS AHAP program. Point Resolution: An estimate made in the field of the aerial extent of the GCP feature. For example, the sharp corner of a roof can often be defined within 2 cm, whereas a road intersection lacks a precise, definable point, and can be resolved in the field, and on imagery, only to about 2 m. Other Notes:Citation: Manley, W. F., Dyurgerov, M., and O'Neel, S., 2001, ANSCIA Barrow Ground Control Point Database, v1: University of Colorado at Boulder, http://instaar.colorado.edu/QGISL/barrow_gcp. Publication Date: October 3, 2001. Contact Email: William.Manley@colorado.edu, INSTAAR QGIS Laboratory. GPS Measurement: GCP static points were measured using Differential GPS. A temporary base station (GCP_ID: C000) was established on the roof of a readily accessible building during GCP measurements. This base operated at least 10 hours each day, and during a continuous 23 hour interval (enabling the Auto-Gipsy solution; see below). The base station and static points were measured with Trimble 4700 dual-frequency, survey-grade receivers. The base station utilized a Trimble compact L1/L2 dual-frequency, survey-grade antenna. Static point measurements were made with a Trimble micro-centered L1/L2 dual-frequency, survey-grade antenna. Static points were measured for at least 15 minutes, with the antenna placed on a tripod, on the ground, or otherwise on the feature. GPS Processing: Horizontal and vertical errors were calculated for each GCP as the RMSE of base station and static point errors. The base station error was quantified using an Auto Gipsy solution (http://www.unavco.ucar.edu/data_support/processing/gipsy/gipsy_info.html). The base station errors are 0.6 cm (horizontal) and 2.5 cm (vertical). Static point errors were calculated using Trimble specifications (Trimble 4700 Operation Manual, version one, revision B, 1998: Sunnyvale, CA). Stated precisions do not account for setup errors (i.e. positioning the antenna relative to the feature being measured). Coordinates are referenced to the ITRF 94 reference frame, a specific subset of WGS84. Due to lack of sufficient information to calibrate this region for the NIMA EGM96 Geoid Model, and high local errors in this model, elevations were calculated and are reported as height above the WGS84 ellipsoid. Based on informal field observation, calculated heights above ellipsoid can be considered reasonably accurate measures of height above mean sea level. Where known, elevations were corrected for antenna height. For GIS purposes, horizontal and vertical datums can be considered WGS84. Access Constraints: None. Use Constraints: Dataset credit required. Use at your own risk. Distribution Liability: Users must assume responsibility to determine the usability of this data for their purposes. 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