Odyssey THEMIS VIS Geometric Projection V2 (VGEO2)
VGEO2 – Visible Geometric Projection V2
Instrument: Thermal Emission Imaging System
PDS Data Set ID: ODY-M-THM-5-VISGEO-V2.0 (in data directories odtvg2_****)DOI: 10.17189/1520386
For more information about all THEMIS GEO products, see the Geometric Processing Guide.
The Thermal Emission Imaging System (THEMIS) Visible Geometric Projection V2 (VGEO2) data products spatially registered, spectral image CUBES derived from the THEMIS calibrated radiance products (VISRDRs). The products are completed by projecting the image into standard Mars coordinates, and then applying any additional image processing. Most of the VGEO1 standard products include the application of a cosmetic filter which renders them undesirable for rigorous spectroscopic studies.
VGEO2 products can be projected in three ways: polar stereographic (POL), sinusoidal (SNU), and equirectangular (EQR) projections. THEMIS images with a center latitude greater than 60 degrees or less than -60 degrees are projected using the polar stereographic standards to give a true perspective of the surface features. The center of each THEMIS polar projection is defined by the image location: the center latitude is set to either +90 or -90 latitude, whichever pole is closest in proximity to the image; the center longitude is set to the “Meridian Longitude” as calculated from the start and end points of the image.
The sinusoidal projection is an equal area, partially equidistant projection where latitude lines are drawn as parallel lines, the center meridian is a straight, vertical line, and the remaining longitude lines curve with increasing distance away from the center.
The equirectangular projection is commonly referred to as the “simple cylindrical” projection when the center latitude is fixed at the equator. In general, the EQR projection is chosen for the ease of use in two dimensional maps of relatively large areas. When used with THEMIS images, however, caution is warranted as these cylindrical projections tend to cause distortion of size, especially at points increasing in distance away from the center of projection. The implementation of the EQR projection is based on the formula for a sphere. The sphere is defined by the radius at the center latitude of the projection. The values for scale and resolution are true at the center latitude of the projection.
To minimize the distortion per image, the center of the EQR projection is based on the observed center of the THEMIS image. The center longitude of the projection is set to the observed center longitude of the image; the center latitude value is based on where the observed center latitude falls within the 5 degree latitude grid shown in the table below. When attempting to mosaic a THEMIS EQR.CUB with another overlapping image, it may be necessary to adjust the center of projection of the THEMIS image to match that of the other image. Furthermore, the user should also consider cropping THEMIS images that are significantly longer than the desired final mosaic.
EQR center latitude grid:
|
Image Latitude Range |
Projection CLAT |
Image Latitude Range |
Projection CLAT |
|
65 to 70 |
65 |
-65 to -70 |
-65 |
|
60 to 65 |
60 |
-60 to -65 |
-60 |
|
55 to 60 |
55 |
-55 to -60 |
-55 |
|
50 to 55 |
50 |
-50 to -55 |
-50 |
|
45 to 50 |
45 |
-45 to -50 |
-45 |
|
40 to 45 |
40 |
-40 to -45 |
-40 |
|
35 to 40 |
35 |
-35 to -40 |
-35 |
|
30 to 35 |
30 |
-30 to -35 |
-30 |
|
25 to 30 |
25 |
-25 to -30 |
-25 |
|
20 to 25 |
20 |
-20 to -25 |
-20 |
|
15 to 20 |
15 |
-15 to -20 |
-15 |
|
10 to 15 |
10 |
-10 to -15 |
-10 |
|
5 to 10 |
5 |
-5 to -10 |
-5 |
|
0 to 5 |
0 |
0 to -5 |
-0 |
THEMIS VIS images are acquired in framelets of size 1024 pixels crosstrack by 192 lines downtrack. The number of framelets is determined by the number of bands selected (five available, enumerated in the table below) and the spatial resolution selected (three summing modes available). If spatial summing is not applied, either a single-band, 19-framelet (65.6 km) image or a 5-band, 3-framelet (10.3 km) image can be collected.
THEMIS available visible bands:
|
Band Numbers |
Center (μm) |
FWHM* (μm) |
|
V-1 |
0.425 |
0.049 |
|
V-2 |
0.540 |
0.051 |
|
V-3 |
0.654 |
0.053 |
|
V-4 |
0.749 |
0.053 |
|
V-5 |
0.860 |
0.045 |
*Full-width, Half-maximum
THEMIS VGEO2 products have the following file names:
VnnnnnsssLOC.CUB (with detached PDS label)
where:
nnnnn = 5-digit mission orbit when the image was collected
sss = 3-digit image sequence number indicating the order that images were collected each orbit (001 = first image collected in the nnnnn orbit)
In ODE, VGEO2 products have the following Product IDs:
VNNNNNSSSLOC