John R. Weirich 23 Jan 2024 Product Description for "Nathair Facula Topography and Photometric Cube Data" This bundle contains topography and photometric data generated by the Gaskell Stereophotoclinometry (SPC) software suite for an area within the Nathair Facula region on Mercury. Details on SPC can be found in Gaskell et al. (2008, 2023). The kernels used to determine the initial spacecraft position and pointing of each image are listed as a text file. The kernels used are "flight kernels", which are unfortunately no longer available to the public, but should be similar to the kernels archived at NAIF (https://naif.jpl.nasa.gov/pub/naif/pds/data/mess-e_v_h-spice-6-v1.0/messsp_1000/). We have not evaluated the kernels at NAIF, but recommend matching the names as similarly as possible. MESSENGER Mercury Dual Imaging System (MDIS) images are used irrespective of the filter. No broadband (i.e. clear) filter images were used to generate the topography. Two locations within Nathair Facula are contained in the bundle, each with a different location, size, and Ground Sample Distance (GSD). For those unfamiliar with GSD, GSD is analogous to resolution of an image, but for topography. A list of locations follows. Nearest Feature Name Center Lat/East Lon Size* (km) GSD (m) Nathair Facula (nafn35066) 34.54/65.62 130 by 130 120 Nathair Facula (nafn36064) 35.85/64.17 80 by 80 60 * For the geoTiff, this is the geodesic distance. See the geoTiff paragraph in the "Formats" section for details. Topography Data We generated regional Digital Terrain Models (DTM) using MDIS images from both the Wide Angle Camera (WAC) and Narrow Angle Camera (NAC). After generation, the topography was aligned to the Nathair Facula DTM published by Fassett et al. (2016). The topography data is provided in both cubes readable by the USGS ISIS program, and as a geoTiff which can be read by programs such as ArcGIS. The topographic error of SPC DTMs was estimated as a part of the OSIRIS-REx mission. As part of NASA Class B software validation, Weirich et al. (2022) used a synthetic, but realistic, digital asteroid with known heights to produce a simulated mission image suite. Note that this test was performed with an early version of the mission image suite, which had less stereo than the flown mission. With only these images, and no access to the synthetic asteroid, we generated an SPC global DTM. This SPC DTM was then compared to the original synthetic DTM to characterize the actual errors. Weirich et al. (2022) showed that a sufficient, but not ideal, image set produced DTMs that were typically accurate to one image pixel of the finest-resolution image. See Weirich et al. (2022) for definitions of a sufficient and ideal image set. Also see Palmer et al. (2022) and Barnouin et al. (2020) for descriptions of good and poor imagery for SPC. The worst vertex in the test DTM was accurate to about three image pixels of the finest-resolution image. Note that these errors are for both the vertical (i.e. radial) and horizontal directions. Unlike the OSIRIS-REx testing, we did not build the DTMs at Nathair Facula with a GSD of the finest-resolution image. So instead of the typical vertex accuracy estimated to be one image pixel of the finest-resolution image, it is instead estimated to be accurate to one DTM GSD. Likewise, the worst DTM vertex is estimated to be accurate to three DTM GSD. A good rule of thumb is to estimate the accuracy of the Nathair Facula DTMs to be one to two DTM GSD. This gives 120 m to 240 m for Nathair Facula (Loc 1) and 60 m to 120 m for Nathair Facula (Loc 2). SPC has a critical internal consistency check that provides an evaluation of the quality of the DTM, specifically the formal uncertainty, or FormU (Weirich et al., 2022). The FormU indicates how closely the data from the model matches the imagery data. While the one to two DTM GSD is the estimated accuracy of an individual vertex determined from a test case (i.e. OSIRIS-REx pre-launch testing), FormU is instead a holistic value for a specific DTM. Nathair Facula (Loc 1) has a FormU of 97.2 m, and Nathair Facula (Loc 2) has a FormU of 73.4 m. Formats Cubes: The cubes are not projected but approximate north up. Orientation vectors are given in the PDS label under local_georeference_information. The cubes are given in "SPC space" so that resampling of the SPC data does not occur. The horizontal spacing between the vertices is the GSD of the DTM. Three cubes fully define the topography. These are topography, latitude, and longitude cubes. The topography cube is the elevation value in meters from Mercury's datum of 2439.4 km, the latitude cube is the Latitude value of that vertex in degrees, and the longitude cube is the East Longitude value of that vertex in degrees. All Cube files end with a "c" to distinguish them from other files. Note that all values in the topography, latitude, and longitude cube files are valid, hence there is not a "NoData" or "missing constant" value defined. However, programs such as GDAL and ArcMap will nonetheless still report a "NoData" value, which is a bogus value. GeoTiff: The geoTiffs are projected and have square pixels. As such, they represent resampled values from the topography generated by SPC, which may affect the accuracy of the heights. In particular, there are arcuate/linear artifacts that can be seen after computing the curvature from the topography and in slope/hillshade products. The SPC data was converted using the Generic Mapping Tool (GMT) program nearneighbor. The program parameters were a search radius of 0.015 degrees, 16 sectors, and a minimum of 8 sectors. If there were not enough valid vertices to meet these parameters, then a NoData value of 4286578688 is written. The data was then projected using the Geospatial Data Abstraction Library (GDAL) program gdalwarp and written to a geoTiff. Since the data is projected onto a sphere and is not in planar format, measuring planar distances will not give the correct distance. Users should be warned that many programs default to measuring planar distances. Likewise, multiplying the GSD by the number of pixels in the x- and y-direction will not give the correct distance. Instead the geodesic distance must be measured, and this value is what is shown in the table at the top of this document. A single geoTiff records the elevation in meters from Mercury's datum of 2439.4 km, as well as the latitude and longitude of each vertex. These files are 32-bit, which is required to obtain the spatial resolutions needed for some solar system bodies. To provide data product similarity across all bodies, we always generate 32-bit geoTiffs even when high spatial resolution is not needed. 32-bit geoTiff files cannot be read by simple programs, and require programs such as ArcGIS to be accessed. All geoTiff files end with a "g" to distinguish them from other files. Photometric Data We generated photometric data (in cube format) that is aligned to and has the same horizontal spacing as the topographic cube data. Fewer images were used for photometric data than for topographic data. For each MDIS image used for photometric data, four cubes were generated; I/F (reflectance), Local Phase Angle, Local Emission Angle, and Local Incidence Angle. Part of the SPC process is to align each image to the topography by adjusting the position and pointing of the spacecraft. The alignment is typically accurate to the size of 1 GSD or better, though rarely for some images in localized areas the misalignment may be as large as 3 or 4 GSD. All angles were determined by the Sun and spacecraft position relative to the height of each DTM vertex. All cubes of a particular location are in the same physical space. In other words, sample,line (100,100) of the topography cube maps to sample,line (100,100) in all the other cubes giving I/F, local phase, incidence, and emission angles for all images. Note that while some cubes have values for all pixels, only pixels that correspond to non-zero values in the reflectance cube (i.e. I/F or "ioverf") are valid. Sample/line values that are zero in the reflectance cube are also invalid for the phase, emission, and incidence angles. File Naming Schema All filenames in this bundle begin with a identifier. This identifier has both region and central lat/lon DTM information. The first three characters will be of an abbreviated version of the region, the next three for the latitude, and the next three for longitude. The latitude is represented by an "n" for North and an "s" for South, followed by the numeric value of the latitude accurate to a whole number. The longitude is represented by the numeric value in East Longitude accurate to a whole number. Location examples: Nathair Facula 34.54 Lat, 65.62 E Lon becomes nafn35066 For photometric data, the next identifier is the last five digits of the MDIS image name, and the filter name. So EW0219648419G becomes 48419g. The next identifier is , which will be "topo" for topography, "lat" for latitude, "lon" for longitude, "ioverf" for I/F, "a" for phase, "e" for emission, and "i" for incidence. The last identifier is one character to make the cube and geoTiff xml labels unique. "c" will be used for cubes, while "g" will be for geoTiff. Thus, all topography, latitude, and longitude cubes will have the filename structure: __c.cub All topography geoTiffs will have the filename structure: _topo_g.tif All photometric cubes will have the filename structure: ___c.cub Locations This bundle contains two DTMs for one region. ---------------------------------- Nathair Facula: This location was constructed to analyze the volcanic vent and associated pyroclastic deposits to understand the style of emplacement, eruption volume, and eruption conditions. Location at 34.54 Lat, 65.62 E Lon has a 120 m GSD. 463 images used to generate the topography, found in nafn35066_imglist.txt 26 images in cubes EW0219648419G EW0234579229G EW0234579346G EW0234621789G EW0234664220G EW0239329993G EW0239664247G EW0249757808G EW0249786604G EW0249815393G EW0249815484G EW0249844198G EW0249844302G EW0249959465G EW0254884916G EW0254913709G EW1003815006G EW1003872619G EW1013954709G EW1013954769G EW1013983574G EW1014012330G EW1014012379G EW1016433646G EW1016462487G EW1028991509G Location at 35.85 Lat, 64.17 E Lon has 60 m GSD. 413 image used to generate the topography, found in nafn36064_imglist.txt 37 images in cubes EN0224508408M EN0224508427M EN0224508446M EN0239497195M EN0239497209M EN0239497223M EN0239497237M EN0239664282M EN0239664296M EN0239664310M EN1003815045M EN1003815055M EN1003815065M EN1003815075M EN1003843846M EN1003843856M EN1003843866M EN1003843876M EN1003930266M EN1003930270M EN1003930274M EN1003930278M EN1003930282M EN1014444082M EN1016174383M EN1016174574M EN1016318329M EN1016318339M EN1016318349M EN1016318359M EN1049462354M EN1049462358M EN1059413252M EN1059413264M EN1059413270M EW0221843895G EW0237041886G References: Barnouin, O., Daly, M., Palmer, E., et al., Digital terrain mapping by the OSIRIS-REx mission, Planetary and Space Science, Vol. 180, id 104764, 2020. https://doi.org/10.1016/j.pss.2019.104764 Fassett, C., Ames stereo pipeline-derived digital terrain models of Mercury from MESSENGER stereo imaging, Planetary and Space Science, Vol 134, p. 19-28, 2016. https://doi.org/10.1016/j.pss.2016.10.001 Gaskell, R.W., Barnouin-Jha, O., Scheeres, D.J., et al., Characterizing and navigating small bodies with imaging data, Meteoritics and Planetary Science, 43, Nr 6, p. 1049-1061, 2008. https://doi.org/10.1111/j.1945-5100.2008.tb00692.x Gaskell, R.W., Barnouin, O.S., Daly, M.G., Palmer, E.E., Weirich, J.R., Ernst, C.M., Daly, R.T., Lauretta, D.S., Stereophotoclinometry on the OSIRIS-REx Mission: Mathematics and Methods. Planetary Science Journal, Vol. 4, No. 4, id 63, 15pp., 2023. https://doi.org/10.3847/PSJ/acc4b9 Palmer, E.E., Gaskell, R., Daly, M.G., Barnouin, O.S., Adam, C.D. and Lauretta, D.S., Practical Stereophotoclinometry for Modeling Shape and Topography on Planetary Missions. Planetary Science Journal, Vol. 3, No. 5, id 102, 16 pp., 2022. https://doi.org/10.3847/PSJ/ac460f Weirich, J.R., Palmer, E.E, Daly, M.G., Barnouin, O.S., Getzandanner, K., Kidd, J.N., Adam, C.D., Gaskell, R., Lauretta, D.S., Quality Assessment of Stereophotoclinometry as a Shape Modeling Method Using a Synthetic Asteroid. Planetary Science Journal, Vol. 3, No. 5, id 103, 12 pp., 2022. https://doi.org/10.3847/PSJ/ac46d2