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HASSELBLAD 500 EL/M 70-MM CAMERAS	NASA has modified standard off-the-shelf Hasselblad 70 mm cameras to operate in zero gravity aboard the Space Shuttle. A data recording module (DRM) has also been installed on each camera to record the date, time (Greenwich Mean Time), mission number, roll number, and frame number. The cameras utilize a 70 mm film format and operate with one of three lenses (50, 100, or 250 mm) to acquire high quality photographs through four viewing ports on the Shuttle.
HDT	High Density Tapes HDTs are high density (high capacity) magnetic tapes.
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HORIZONTAL INTEGRATION	The process of mosaicking adjacent parts of a map or image together into a single map or image. This process might require geometric adjustments to the image itself or the features within it so that matching occurs across mosaic seams.
HORIZONTAL POLARIZATION	Transmission of microwaves so that the electric lines of force are horizontal, while the magnetic lines of force are vertical.
HORIZONTAL POSITIONAL ACCURACY	Horizontal positional accuracy is based upon the use of USGS source quadrangles which are compiled to meet National Map Accuracy Standards (NMAS). NMAS horizontal accuracy requires that at least 90 percent of points tested are within 0.02 inches of the true position. The digital data are estimated to contain a horizontal positional error of less than or equal to 0.003 inches standard error in the two component directions relative to the source quadrangle.
HRPT	High Resolution Picture Transmission HRPT data are full resolution image data transmitted to a ground station as they are collected. The average instantaneous field-of-view of 1.4 milliradians yields a HRPT ground resolution of approximately 1.1 km at the satellite nadir from the nominal orbit altitude of 833 km (517 mi).
HRV	High Resolution Visible Imaging Instrument The HRV instrument is a multispectral radiometer designed for SPOT spacecraft. The HRV instrument provides for high-resolution imaging in the visible and near-infrared portions of the electromagnetic spectrum. The first three SPOT satellites carry twin HRVs that operate in a number of viewing configurations and in different spectral modes. Some of those viewing configurations and spectral modes include one HRV only operating in a dual spectral mode (i.e., in both panchromatic mode and multispectral mode); two HRVs operating in the twin- viewing configuration (i.e., one HRV in panchromatic mode and one HRV in multispectral mode); and two HRVs operating independently of each other (i.e., not in twin-viewing configuration).
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HYDROLOGY	Scientific study of the waters of the Earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of ground water.
HYPERTEXT	The use of links between related words, graphics, or documents that allow the user to jump to associated topics or definitions when reading menus or help files.
HYPSOGRAPHY	The scientific study of the Earth's topologic configuration above sea level, especially the measurement and mapping of land elevation.