Enceladus Cassini ISS Gridded Topography 2.2km v1
This 2.5D topographic model of Enceladus was created by first generating a fully 3D shape (radius) model from a dense network of image tie points and photogrammetrically solving for point latitude, longitude, and radius. We then calculated a best-fit triaxial ellipsoid and subtracted it from the shape model to determine the local deviation from the ellipsoid (i.e., the topography). The resulting values were interpolated to a regular grid at 2 pixels per degree (2.2 km/pixel at the equator). The model is both near-global in extent and relatively high "resolution", and is intended to support scientific investigations, future mission planning activities, and generation of improved orthoimages.
The global shape model was created following the conceptual approach applied to Mercury MESSENGER data by Becker et al. (2016). We first established a dense network of image tie points and photogrammetrically solved for point latitude, longitude, and radius using a least squares bundle adjustment. The resulting point cloud was filtered to exclude points with a minimum expected vertical precision (EP) greater than 2 km, and then interpolated to a regularly gridded 2.5D global shape model.
Archinal, B. A., Acton, C. H., A'Hearn, M. F., Conrad, A., Consolmagno, G. J., Duxbury, T., Hilton, J. L., et al. (2015). Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015. Celestial Mechanics and Dynamical Astronomy 130(22). https://doi.org/10.10007/s10569-017-9805-5
Becker, K. J., Robinsons, M. S., Becker, T. L., Weller, L. A., Edmundson, K. L., Neumann, G. A., Perry, M. E., & Solomon, S. C. (2016). First global digital elevation model of Mercury. Paper presented at the 47th Lunar and Planetary Science Conference, Lunar and Planetary Institute, Houston, TX. https://www.hou.usra.edu/meetings/lpsc2016/pdf/2959.pdf
Beyer, R. A., Alexandrov, O, & McMichael, S. (2018). The Ames Stereo Pipeline: NASA’s open source software for deriving and processing terrain data. Earth and Space Science, 5(9). https://doi.org/10.1029/2018EA000409
Bland, M. T., Becker, T. L., Edmundson, K. L., Roatsch, T., Archinal, B. A., Takir, D., Patterson, G. W., et al. (2018). A new Enceladus global control network, image mosaic, and updated pointing kernels from Cassini's 13‐year mission. Earth and Space Science, 5(10), 604–621. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018EA000399
Bland, M. T., Weller, L. A., Mayer, D. P., & Archinal, B. A. (2020). A global shape model for Saturn's moon Enceladus from a dense photogrammetric control network. Annals of the International Society for Photogrammetry and Remote Sensing and Spatial Information Sciences, 3, 579-586. https://doi.org/10.5194/isprs-annals-V-3-2020-579-2020
Porco, C. C., West, R. A., Squyres, S., McEwen, A., Thomas, P., Murray, C. D., Delgenio, A., et al. (2004). Cassini Imaging Science: Instrument characteristics and anticipated scientific investigations at Saturn. Space Science Review, 115, 363-497. https://doi.org/10.1007/s11214-004-1456-7
Schenk, P. M. & McKinnon, W. B. (2009). One-hundred-km-scale basins on Enceladus: Evidence for an active shell. Geophysical Research Letters, 36(L16202). https://doi.org/10.1029/2009GL039916
Tajeddine, R., Soderlund, K. M., Thomas, P. C., Helfenstein, P., Hedman, M. M., Burns, J. A., & Schenk, P. M. (2017). True polar wander of Enceladus from topographic data. Icarus, 295, 46-60. https://doi.org/10.1016/j.icarus.2017.04.019
- Publication Date
- 28 August 2020
- Michael T. Bland
- Lynn A. Weller, David P. Mayer, Brent A. Archinal
- Added to Astropedia
- 28 August 2020
- 11 May 2021
Previous shape models of Enceladus have been generated from either stereo imaging (high resolution but limited spatial extent, e.g., Schenk and McKinnon ) or limb topography (global extent but lower resolution, e.g., Tajeddine et al. ). This work provides a new topography model that is both near-global in extent and relatively high "resolution." The model is intended to support scientific investigations, future mission planning activities, and generation of improved orthoimages.
- Geospatial Data Presentation Form
- Topographic Map
- Online Linkage
- Native Data Set Environment
- ISIS v3
- Supplemental Information
- https://pds-imaging.jpl.nasa.gov/portal/cassini_mission.html, https://astrogeology.usgs.gov/search/map/Enceladus/Cassini/Enceladus_Cassini_ISS_Global_Mosaic_100m_HPF
Data Status and Quality
- Time Period of Content Begin
- 6 October 2004
- Time Period of Content End
- 9 September 2016
- Currentness Reference
- Ground condition
- Update Frequency
- None planned
- Logical Consistency Report
- The images used in the photogrammetric solution are relatively controlled. We fixed the position of the crater Salih at its IAU defined longitude of 5° [Archinal et al. 2015].
- Completeness Report
The density of the underlying 3D point cloud is highly variable, but sufficient to provide 92.5% coverage in a product gridded at 2 pixels/degree (with an average of 10.6 points per pixel) or 96.3% coverage when gridded at 1 pixel/degree (with an average of 42 points per pixel). When gridded such that each pixel contains, on average, 1 point, the coverage is 57%. See the "Count" products in the ancillary files accompanying the Gridded Radius product and Bland et al.  for more detail. The Cassini ISS images used in the control network are a subset of those available and include nearly all of the images (625 in total) from the CLR, GRN, IR3, and UV3 filters with pixel scale from 50-500 m/pixel, phase angle less than 120 degrees. However, several images that meet this criterion were not included. A complete list of images is provided in Bland et al. . This point cloud was filtered to remove points with expected vertical precision (EP) larger than 2 km, which is the known topographic variability of Enceladus.
- Process Description
The gridded topography product was derived from our 3D point cloud, which was generated from a dense photogrammetric control network of more than 600 moderate- to high-resolution images from the NASA Cassini mission's Imaging Science Subsystem (ISS). The Cassini ISS images used in the control network are a subset of those available and include nearly all of the images (625 in total) from the CLR, GRN, IR3, and UV3 filters with pixel scale from 50-500 m/pixel, phase angle less than 120 degrees. However, several images that meet this criterion were not included. A complete list of images is provided in Bland et al. . The control network was developed using the USGS' Integrated Software for Imagers and Spectrometers (ISIS) and consists of 892, 457 points and more than 30 million image measures. Bundle adjustment of the network was performed using the ISIS jigsaw application, resulting in adjusted latitude, longitude, and radius for each point. The shape model is described more fully in Bland et al. .
With the 3D shape model created, we derived the best-fit triaxial ellipsoid using ISIS’s jigsaw application to solve for Enceladus’ body parameters. The resulting radii values were 256.3 km, 251.2 km, 248.3 km, for the a, b, and c axis, respectively. The values are consistent with IAU values. For each latitude/longitude point in the shape model we calculated the best-fit ellipsoidal radius and then subtracted it from the actual measured radius to determine the local deviation from the ellipsoid at that point. This deviation is the topography. The result was a 3D point cloud of latitude, longitude, and topography. The gridded product was derived by interpolating the 3D point cloud to a regular grid using the Ames Stereo Pipeline's point2dem application (Beyer et al. 2018). Before interpolation, the 3D point cloud was filtered to remove points with expected vertical precision (EP) larger than 2 km, which is the known topographic variability of Enceladus. We chose a grid spacing of 2 pixel/degree based on an analysis of areal coverage and point density (points per pixel). Values at each pixel in the gridded product were determined by a Gaussian weighted average of the points within each pixel. We used a 1-pixel search window in point2dem, which eliminates any smoothing that results from including points outside the pixel in the average. The product is thus spatially consistent with our Gridded Radius product.
Horizontal Positional Accuracy Value: 37 m relative. Least square bundle adjustment of the photogrammetric control network resulted in root mean square (RMS) residuals of 37 m and 36 m in latitude and longitude, respectively. The total RMS uncertainty (including radius) in image locations was 0.3 pixels. However, because Enceladus does not have a well-defined reference system, these accuracies are not absolute. We attempted to achieve some degree of absolute accuracy by fixing the position of the crater Salih at its IAU defined longitude of 5o [Archinal et al. 2015].
Vertical Positional Accuracy Value: 29 m to 2 km. We assess the vertical accuracy in two ways. The least square photogrammetric bundle solution resulted in an RMS uncertainty in point radius of 57 m. We also evaluate the stereo quality of each point by calculating the minimum expected vertical precision (EP) of each point, where EP = rho*GSD/(b/h) with GSD the ground sample distance, (b/h) the base to height ratio of the stereo image pair, and rho the quality matching in fractions of a pixel. We conservatively take rho=1 (values of rho are typically 0.2-0.3 for high-quality imaging), which likely results in EP values that are too large. Even so, EP ranges from as low as 29 m, to as high as 2 km. EP is highly spatially variable.
- Horizontal Positional Accuracy Value
- Horizontal Positional Accuracy Report
- Accurate to Control Net
- Vertical Positional Accuracy Value
- Vertical Positional Accuracy Report
- Accurate to Control Net
- Entity and Attribute Overview
- Elevation relative to a triaxial ellipsoid with radii of 256.3 km, 251.2 km, 248.3 km (i.e., the best-fit ellipsoid to our shape model).
- Entity and Attribute Detailed Description
- Elevation values were computed by subtracting the best-fit ellipsoid from the radius value at each point. Values were then interpolated to a regular grid.
- PDS Status
- PDS 4 Like
- Source PDS Archive
- Source Online Linkage
- https://pds-imaging.jpl.nasa.gov/portal/cassini_mission.html, https://pds-imaging.jpl.nasa.gov/volumes/iss.html
- Attribute Accuracy Report
- Accurate to Control Net
- Minimum Latitude
- Maximum Latitude
- Minimum Longitude
- Maximum Longitude
- Direct Spatial Reference Method
- Object Type
- Lines (pixels)
- Samples (pixels)
- Bit Type
- Radius A
- Radius C
- Pixel Resolution (meters/pixel)
- Scale (pixels/degree)
- Horizontal Coordinate System Units
- Map Projection Name
- Latitude Type
- Longitude Direction
- Positive East
- Longitude Domain
- 0 to 360