LROC Special Data Record WAC Digital Terrain Map (SDWDTM)
SDWDTM – Special Data Record WAC Digital Terrain Map
Instrument: Lunar Reconnaissance Orbiter Camera
PDS Data Set ID: LRO-L-LROC-5-RDR-V1.0
For more information about LROC Special products, see the RDR Product SIS.
For more information about all LROC products, see the LROC Archive Volume SIS.
The Lunar Reconnaissance Orbiter Camera (LROC) Special Data Record WAC Digital Terrain Map (SDWDTM) data product consists of a set of images produced from Wide Angle Camera (WAC) images with appropriate illumination and viewing geometries suitable for geometric or photometric stereo research and development activities. Products are created on a best-effort basis and are intended to demonstrate the utility of the data collection and the methodology employed in the derivation of high-resolution Digital Elevation Models (DEMs) through photometric stereo techniques. Data products are grouped together by the locality they cover.
Photometric stereo is well known to the artificial intelligence and robotic vision community but not in the planetary science community. Photoclinometry from single images (or shape from shading) is well known, but the problem is unconstrained. The major unknowns are slope, slope azimuth, and the albedo of the surface. Photometric stereo resolves these ambiguities from the use of two or more images of the scene from nearly the same viewing geometry but different illumination angles. Planetary scientists have rarely exploited this technique because few missions have provided more than a rare set of appropriate images.
Even when albedo is uniform there are often two solutions for slope and slope azimuth. This ambiguity can be resolved via use of a 3rd image acquired with different (not co-planar) illumination; albedo can be extracted as well. The solution can be obtained rapidly by creating a three-dimensional lookup table for each triplet of images in which the calibrated intensity of each image is quantized to form the indices, slope, slope azimuth, and albedo are calculated for each triplet of intensities with the photometric function. The slope and slope azimuth solutions form vectors or a “needle diagram” in the three-dimensional table, and albedo is defined by position along the vector. The three images for photometric stereo must be co-registered to sub-pixel accuracy to obtain reliable results at the scale of single pixels. If registration is poor then pixels will be binned to minimize spurious results.
LRO’s orbit is ideal for the acquisition of triplets of images appropriate for photometric stereo with the best lighting geometry coming from the latitude bands near ± 30°-70° because image triplets with non-coplanar illumination can be easily acquired and there are few large shadows except in the early morning and late afternoon.
Once the output files of slope and slope azimuth are created, it is possible to derive relative elevations by integrating slopes across the image. Low-frequency topography can be corrected to match the best available altimetry values over the scene thereby providing high-resolution absolute elevations relative to the center of mass.
There are two types of SDWDTM files: color shaded relief images (3-channel RGB TIFF, 8 bits/sample, 3 samples/pixel in single image plane configuration) and the images from which the Global Lunar DTM 100m topographic model (GLD100) is derived.
LROC SDWDTM color shaded relief products have the following file names:
WAC_CSHADE_taaadoooo_rrrP.TIF (with detached PDS label)
where:
t = projection type
E = equirectangular
O = orthographic
P = polar stereographic
aaa = latitude center to the tenth of a degree (nn.n)
d = direction
N = north
S = south
oooo = longitude center to the tenth of a degree (nnn.n)
rrr = 3-digit resolution
LROC SDWDTM color shaded relief products with an overlain grid have the following file names:
WAC_CSHADE_taaadoooo_rrrP_GRID.TIF
where:
taaadoooo_rrrP = product name from above
The data set also contains a scale image for interpreting the color shaded relief products. The image is named WAC_CSHADE_LEGEND.TIF, with its corresponding label file.
LROC SDWDTM GLD100 image products have the following file names:
WAC_GLD100_taaadoooo_rrru.IMG
where:
t = projection type
E = equirectangular
O = orthographic
P = polar stereographic
aaa = latitude center to the tenth of a degree (nn.n)
d = direction
N = north
S = south
oooo = longitude center to the tenth of a degree (nnn.n)
rrr = 3-digit resolution
u = resolution units
M = meters/pixel
P = pixels/degree
In ODE, SDWDTM products have one of the following product IDs:
WAC_CSHADE_TAAADOOOO_RRRP
WAC_CSHADE_TAAADOOOO_RRRP_GRID
WAC_GLD100_TAAADOOOO_RRRU