Odyssey THEMIS IR Geometric Projection V2 (IRGEO2)
IRGEO2 – Infrared Geometric Projection V2
Instrument: Thermal Emission Imaging System
PDS Data Set ID: ODY-M-THM-5-IRGEO-V2.0 (in data directories odtig2_****)DOI: 10.17189/1520213
For more information about all THEMIS GEO products, see the Geometric Processing Guide.
The Thermal Emission Imaging System (THEMIS) Infrared Geometric Projection V1 (IRGEO1) data products are spatially registered, spectral image CUBEs derived from the THEMIS calibrated radiance products (IRRDRs). Because the products contain geometrically registered and atmospherically corrected calibrated radiance, IRGEO1 products are ideal for use in surface studies and for use with other projected Mars datasets.
IRGEO2 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.QUB 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 IR images are acquired at selectable image lengths and in combinations of ten selectable bands (enumerated in the table below). The image width is 320 pixels (32 km, based on the nominal 400 km mapping orbit) and the length is variable, in multiples of 256 line increments, with a minimum and maximum image lengths of 272 and 65,296 lines respectively (27.2 km and 6,530 km, based on the nominal mapping orbit). The IR focal plane is covered by ten ~1 μm-bandwidth strip filters, producing ten band images with bands 1 and 2 having the same wavelength range.
THEMIS available infrared bands:
|
Band Numbers |
Center (μm) |
FWHM* (μm) |
|
IR-1 |
6.78 |
1.01 |
|
IR-2 |
6.78 |
1.01 |
|
IR-3 |
7.93 |
1.09 |
|
IR-4 |
8.56 |
1.16 |
|
IR-5 |
9.35 |
1.20 |
|
IR-6 |
10.21 |
1.10 |
|
IR-7 |
11.04 |
1.19 |
|
IR-8 |
11.79 |
1.07 |
|
IR-9 |
12.57 |
0.81 |
|
IR-10 |
14.88 |
0.87 |
*Full-width, Half-maximum
THEMIS IRGEO2 products have the following file names:
InnnnnsssSNU.CBU.gz (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, IRGEO2 products have the following Product IDs:
INNNNNSSSSNU