Geography of Mars

• See Viewgraphs: "Venturing into space."
• "Resolution" is one of those terms that seem intuitively obvious on first encountering it, but turns out to have quite an array of dimensions or aspects. There are several kinds of "resolution" that impact what you can detect and discern in imagery and spectroscopy.
• Spatial
  • This is probably the most common meaning. It has to do with the size of the smallest object that can be detected on a target surface, which is a function of the angular resolution of the picture elements (pixels) in a raster image, the sensor's distance to its target, and the resulting instantaneous field of view. It can vary over quite a range, depending on the design and purpose of the instrument:
    • fine or very high resolution: 0.5-5 m (e.g., on Earth: WorldView-3 at 31 cm; Mars Reconnaisance Orbiter HiRISE camera at 30 cm)
    • moderate resolution: 5 m to 100 m (e.g., on Earth, Landsat at 15-30 m; Mars Odyssey Themis visual and infrared camera at 18 to 100 m
    • medium resolution: 100 m to 1 km (e.g., NASA/NOAA/DoD Suomi National Polar-orbiting Partnership (Suomi NPP) Visible Infrared Imaging Radiometer Suite (VIIRS) at 375 and 750 m, depending on band; on Mars Global Surveyor, the red and blue wide-angle Mars Orbiter Camera (MOC) at 240 m)
    • coarse or low resolution: > 1 km (e.g., on Earth: MODIS with 1 km resolution) or NOAA's GOES' 1 - 8 km resolution, depending on channel); NASA MAVEN's IUVS at 200 km
  • Spatial resolution may be variable, as in a descending probe (e.g., Huygens' imagery of Titan's landscapes on the way down) or, on Mars, the Curiosity Rover's Mars Descent Imager (MARDI) or in a satellite system that has an extremely elliptical orbit, such as ISRO's Mars Orbital Mission (MOM) Mars Colour Camera (MCC), which produces imagery varying from 19 m to 4 km
• Vertical
  • Vertical resolution is generally worse than horizontal, as you know if you've taken GPS units into the field and compared readings among units
  • This z coördinate is the basis of digital elevation models (and, if you've taken any of Dr. Wechsler's classes, you're aware of the uncertainty issue)
  • Bases for elevation extraction include laser altimeters, interferometric synthetic aperture radar (SAR), and stereo pairing of images to be analyzed visually by a person
  • The Mars Global Surveyor MOLA instrument you met in Lab 2 had 37.5 cm vertical resolution from shot to shot of the laser, but the uncertainties inherent in orbit reconstruction yielded accuracy variations up to 10 m)
• Spectral
  • The electromagnetic spectrum is divided into various named bands divided by wavelength or frequency, as seen here.
  • Sensors are designed to be sensitive to certain bandwidths, perhaps in visible light, infrared, radio, or ultraviolet
  • Panchromatic (all bands within a large range, often fine resolution, e.g., Landsat ETM+ processed in software)
  • Multispectral (3-100 or so bands, at discrete intervals along the spectrum but gaps in between these). Examples include the 3 band SPOT HRV sensor, the 4 band IKONOS imager, and the 7 band Landsat TM. Sometimes, the ones that have many bands (30-100), each of which is fairly narrow, are called "superspectral" (e.g., MODIS
  • Hyperspectral (16-220 narrow bands contiguous to one another over a spectral range). An example is AVIRIS on Earth and Mars Express' OMEGA imaging spectrometer
• Radiometric
  • Range of intensity values a sensor can detect
  • Basically a function of the number of bits per byte per pixel, as well as the noise in the signal.
    • a 6-bit byte would distinguish 2⁶ levels or 64 different levels on its radiometric scale (e.g., the Multispectral Scanner on Landsat 1, 2, and 3)
    • an 8-bit byte would distinguish 2⁸ levels or 256 (e.g., Landsat 4 and 5, both the MSS and the Thematic Mapper, and SPOT's High Resolution Visible instrument)
    • the MODIS (MODerate rqdesolution Imaging Spectroradiometer) on the Terra and Aqua orbiters here on Earth has a 12 bit byte! This is 2¹² or 4,096 levels!
• Directional
  • Surfaces can produce different radiometric values in a bandwidth depending on the angle of incident illumination and the angle of viewing by the sensor (think of dark blue-grey-green ocean water at noon and blindingly white reflection off ocean water at sunrise or sunset)
  • Different incidence angles can alter scattering or absorbption by dust, gas, or clouds in the atmosphere
  • Some sensors are designed to look, not at the nadir directly below, but at oblique angles, looking forward or backward, for example, which not only accentuates geometric distortion effects but also bi-directional differences in radiometric readings by bandwidth
• Temporal
  • One time (e.g., flyby)
  • Intermittant (e.g., AVIRIS)
  • Repetitive (stationary orbits, e.g., GOES, or regular overflights, e.g., Landsat) and the various Mars orbiters

• Liew, S.C. 2001. Principles of Remote Sensing. Available at http://crisp.nus.edu.sg/~research/tutorial/rsmain.htm
• Natural Resources Canada. 2015. Tutorial: Fundamentals of Remote Sensing. Available at http://www.nrcan.gc.ca/earth- sciences/geomatics/satellite-imagery-air-photos/satellite-imagery- products/educational-resources/9309
• Nicholar M. Short. 2006. The Remote Sensing Tutorial. Available at http://fas.org/irp/imint/docs/rst/

Something else while I'm thinking about it: scale

• "Scale" is one of those concepts that can be counter-intuitive.
• In cartography, it refers to the amount of detail, which is limited by the density of symbolization possible on a given size map.
  • "Large scale" means "large amount of detail" means small area mapped
  • "Small scale" means "small amount of detail" meaning a highly generalized map of a large area
• This is maddeningly easy to get twisted around on.