IMAGE OBJECT | IMAGE_MAP_PROJECTION OBJECT
Support of PDS Topographic Information in STK Products
The Planetary Data System (PDS) was developed and is maintained by JPL for the purpose of establishing standard formats for data sharing between organizations. Many types of data, including topography, are shared in PDS-specified formats.
Details on the STK implementation of the PDS image format for specification of topography follows.
PDS topographic data sets consist of two parts:
- A label (.lbl) file that provides descriptive information in ASCII text to allow for proper interpretation of the image file (.img).
- An image file (.img), which contains the actual raw data.
The label and image files are required to be in the same directory and have the same filename, with different extensions.
The label file, which consists of a set of keyword value pairs, typically contains three sections:
- General information, which usually contains the name of the body of interest.
- An image block, which describes the amount and type of data.
- An image map projection block, which describes the projection and spacing of the data.
Since PDS is an extremely broad standard, STK products do not support the full range of documented options and require that certain keywords be present for proper use of the data sets.
STK products do not currently support the unit qualifiers which may be appended to value fields in PDS label files. The expected units are noted below for all supported keywords.
The use of topography data for bodies other than the Earth may require modifications to the reference shape of the body for accurate determination of the body surface. For more information, see Using Topography Data for Central Bodies Other than the Earth.
PDS Description Documents
See the following for additional information on the PDS format:
General Information
A typical general information section, from a Mars topography data set, is included below for reference.
PDS_VERSION_ID
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=
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PDS3
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RECORD_TYPE
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=
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FIXED_LENGTH
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FILE_RECORDS
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=
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10240
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RECORD_BYTES
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=
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20480
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^IMAGE
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=
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"MEGT_S_128_1.IMG" |
DATA_SET_ID
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"MGS-M-MOLA-5-MEGDR-L3-V1.0"
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PRODUCT_ID
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"MEGT_S_128_1.IMG"
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SPACECRAFT_NAME
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"MARS GLOBAL SURVEYOR"
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INSTRUMENT_ID
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=
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MOLA
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INSTRUMENT_NAME
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"MARS ORBITER LASER ALTIMETER"
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TARGET_NAME
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MARS
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START_TIME
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1997-09-15T19:10:00.000
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STOP_TIME
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2001-06-30T11:10:40.000
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START_ORBIT_NUMBER
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3
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STOP_ORBIT_NUMBER
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20333
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PRODUCT_CREATION_TIME
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2004-07-21T01:00:00
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PRODUCT_VERSION_ID
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"1.0" |
PRODUCER_ID
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MGS_MOLA_TEAM
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PRODUCER_FULL_NAME
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"DAVID E. SMITH"
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PRODUCER_INSTITUTION_NAME
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"GODDARD SPACE FLIGHT CENTER"
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DESCRIPTION
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"This data product is a topographic
map of the south pole of Mars at a resolution of 128 pixels per degree,
based on altimetry data acquired by the Mars Global Surveyor MOLA
instrument and accumulated over the course of the primary and extended
mission. The MOLA Precision Experiment Data Records (PEDRs) are the
source for this data set. Map coordinates use
the IAU2000 reference system and the areoid defined by the Goddard Mars
potential model GMM3 (mgm1025) evaluated to degree and order 50. The
binned data include all MOLA nadir observations from the Mapping Phase
through the Primary and Extended missions from the end of aerobraking
in February 1999 through June 2001."
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STK products only look for a small number of keywords from the general data section of the PDS label file. Those keywords are highlighted in BOLD above, all keywords not in bold or not listed are ignored. Note that although the DESCRIPTION field is not used by AGI products, it sometimes contains information required for proper use of the data set.
^IMAGE
The ^IMAGE keyword is used to specify the name of the image file. The image file must reside in the same directory as the label file. AGI products first look for a file of the same name as the label file, but with the .img extension. If that file is not found, the file specified via the ^IMAGE keyword is used.
TARGET_NAME
The TARGET_NAME keyword is used to specify the object or area of interest for the dataset. AGI products require that the TARGET_NAME be used to specify the name of the central body (MOON, MARS, etc.) to which the topography will be applied.
IMAGE OBJECT
The image object serves to describe the data in the image file. A typical image block is included below:
OBJECT
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IMAGE
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NAME
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=
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MEDIAN_TOPOGRAPHY
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LINES
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=
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10240
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LINE_SAMPLES
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10240
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SAMPLE_TYPE
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MSB_UNSIGNED_INTEGER
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SAMPLE_BITS
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16
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UNIT
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=
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METER
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SCALING_FACTOR
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0.25
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OFFSET
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-8000
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INVALID_CONSTANT
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= | 32768
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DESCRIPTION
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"Each sample represents median
observed topography in units of meters relative to the 3396000 meter
datum at the equator. The topographic datum is an equipotential surface
defined by the Goddard Mars potential model GMM3 (mgm1025) evaluated to
degree and order 50, in the IAU2000 reference frame. To obtain the
observed topography value, apply the above scaling factor and offset:
TOPOGRAPHY = (STORED VALUE * SCALING_FACTOR) + OFFSET."
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END_OBJECT
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IMAGE
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AGI products utilize many of the keywords from the image block of the PDS label file. Those keywords are highlighted in BOLD above; all keywords not in bold or not listed are ignored. Note that although the DESCRIPTION field is not used by AGI products, it sometimes contains information required for proper use of the data set. Note that the UNIT keyword is not used. AGI products assume that the value generated by scaling the stored values and adding the offset is in meters.
NAME
The NAME keyword is not well defined in the PDS documentation, but has been used in a number of PDS data sets that we have seen to specify an interpretation of the data. In particular, the values of MEAN_RADIUS and MEDIAN_TOPOGRAPHY have been observed. AGI products treat the data as topography data if this keyword is missing or has any value other than MEAN_RADIUS. In the case where the MEAN_RADIUS value is specified, an internal transformation is used to compute the equivalent topographic information based on the defined shape of the central body.
LINES
The LINES keyword specifies the number of records in the image file.
LINE_SAMPLES
The LINE_SAMPLES keyword specifies the number of post values per line.
SAMPLE_TYPE
The SAMPLE_TYPE keyword specifies the type of binary data storage used for each value in the image file. AGI products support the following current options: IEEE_REAL, LSB_INTEGER, LSB_UNSIGNED_INTEGER, MSB_INTEGER, MSB_UNSIGNED_INTEGER, PC_REAL. AGI products also support the following obsolete options: INTEGER, UNSIGNED_INTEGER. AGI products automatically adjust the endianness of the data when necessary.
SAMPLE_BITS
The SAMPLE_BITS keyword is used in conjunction with the SAMPLE_TYPE keyword to completely define the binary representation of the data. AGI products support the following options: 8, 16, and 32.
UNIT
The UNIT keyword is used to specify the units of the terrain values after application of the scaling factor. Note that the offset is also assumed to be of this unit. Recognized values are KILOMETER, METER, CENTIMETER and MILLIMETER. The altitude of the topography of a given post in the specified units is computed as:
Alt = Stored_Value * SCALING_FACTOR + OFFSET.
SCALING_FACTOR
The SCALING_FACTOR specifies a scaling of the stored data values to the specified units. AGI products assume that the resulting units are meters unless specified otherwise using the UNIT keyword.
OFFSET
The OFFSET specifies an additive component to the scaled value. AGI products assume that the resulting units are meters unless specified otherwise using the UNIT keyword.
INVALID_CONSTANT
The INVALID_CONSTANT is used to specify a NULL value for the dataset. NULL values are sometimes used to indicate that no data was available for specific pixels of the image. When these values are part of a topography dataset, they are interpreted as points of zero altitude relative to the reference central body shape.
IMAGE_MAP_PROJECTION OBJECT
The IMAGE_MAP_PROJECTION object serves to describe the map projection of the stored data. Relevant information typically includes the name of the projection, the defining parameters of the projection, and spacing between posts (the resolution of the data). The planetographic extent of the data is also often included, but may not be relevant to data in certain projections. An example IMAGE_MAP_PROJECTION block is shown below:
OBJECT
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IMAGE_MAP_PROJECTION
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^DATA_SET_MAP_PROJECTION
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"DSMAP_POLAR.CAT"
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MAP_PROJECTION_TYPE
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"POLAR STEREOGRAPHIC"
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A_AXIS_RADIUS
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1735 <KM>
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B_AXIS_RADIUS
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1735 <KM>
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C_AXIS_RADIUS
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1735 <KM>
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FIRST_STANDARD_PARALLEL
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'N/A'
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SECOND_STANDARD_PARALLEL
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'N/A'
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POSITIVE_LONGITUDE_DIRECTION
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"EAST"
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CENTER_LATITUDE
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-90.0 <DEGREE>
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CENTER_LONGITUDE
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'N/A'
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REFERENCE_LATITUDE
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'N/A'
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REFERENCE_LONGITUDE
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'N/A'
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LINE_FIRST_PIXEL
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1
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LINE_LAST_PIXEL
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=
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3000
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SAMPLE_FIRST_PIXEL
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1
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SAMPLE_LAST_PIXEL
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=
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3000
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MAP_PROJECTION_ROTATION
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0.0
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MAP_RESOLUTION
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=
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256.0 <PIXEL/DEGREE>
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MAP_SCALE
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0.06668 <KM/PIXEL>
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MAXIMUM_LATITUDE
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-85.333820 <DEGREE>
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MINIMUM_LATITUDE
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-90.000000 <DEGREE>
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WESTERNMOST_LONGITUDE
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=
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'N/A'
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EASTERNMOST_LONGITUDE
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=
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'N/A'
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LINE_PROJECTION_OFFSET
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=
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'N/A'
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SAMPLE_PROJECTION_OFFSET
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=
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'N/A'
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COORDINATE_SYSTEM_TYPE
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"BODY-FIXED ROTATING"
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COORDINATE_SYSTEM_NAME
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"PLANETOCENTRIC"
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END_OBJECT
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IMAGE_MAP_PROJECTION
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AGI products utilize many of the keywords from the IMAGE_MAP_PROJECTION block of the PDS label file. Those keywords are highlighted in BOLD above; all keywords not in bold or not listed are ignored. Note that although the COORDINATE_SYSTEM_TYPE is not currently used by AGI products, BODY-FIXED ROTATING is the only value that makes sense for specification of topographical data.
MAP_PROJECTION_TYPE
The MAP_PROJECTION_TYPE specifies the projection of the stored data. AGI products support the following options: SIMPLE CYLINDRICAL and POLAR STEREOGRAPHIC. STEREOGRAPHIC projections are also supported when the center latitude of the projection is +/- 90 degrees such that the data may be interpreted as being in POLAR STEREOGRAPHIC projection.
A_AXIS_RADIUS / B_AXIS_RADIUS / C_AXIS_RADIUS
The AXIS_RADIUS values specify the reference shape to be used as parameters of the map projection. AGI products assume that all values are specified in kilometers.
POSITIVE_LONGITUDE_DIRECTION
The POSITIVE_LONGITUDE_DIRECTION specifies the direction in which longitude is measured as positive. AGI products only support a value of EAST.
CENTER_LATITUDE
The CENTER_LATITUDE specifies the latitude coordinate at the center of the map projection. AGI products assume the value to be specified in degrees.
CENTER_LONGITUDE
The CENTER_LONGITUDE specifies the longitude coordinate at the center of the map projection. AGI products assume the value to be specified in degrees.
LINE_FIRST_PIXEL / LINE_LAST_PIXEL
The LINE_FIRST_PIXEL and LINE_LAST_PIXEL keywords specify the location of the start and end of the actual image inside the image data file. AGI products only support LINE_FIRST_PIXEL = 1 and LINE_LAST_PIXEL = N where N is the number of lines in the image.
SAMPLE_FIRST_PIXEL / SAMPLE _LAST_PIXEL
The SAMPLE _FIRST_PIXEL and SAMPLE _LAST_PIXEL keywords specify the location of the start and end of the actual image inside the image data file. AGI products only support SAMPLE _FIRST_PIXEL = 1 and SAMPLE _LAST_PIXEL = M where M is the number of post values per line in the image.
MAP_PROJECTION_ROTATION
The MAP_PROJECTION_ROTATION specifies the rotation of the stored image data relative to the indicated map projection. AGI products only support MAP_PROJECTION_ROTATION = 0 unless the map projection is POLAR STEREOGRAPHIC.
MAP_RESOLUTION
The MAP_RESOLUTION specifies the resolution of the post data in terms of pixels per degree. The resolution applies to the center point of the projection. This value is equivalent to the value provided in the MAP_SCALE. See MAP_SCALE for more details.
MAP_SCALE
The MAP_SCALE specifies the resolution of the post data in distance units. The resolution applies to the center point of the projection. The reference point for the SIMPLE CYLINDRICAL projection is at the intersection of the prime meridian and the equator. The reference point for POLAR STEREOGRAPHIC projections is at pole. This value is equivalent to the value provided in the MAP_RESOLUTION. The value provided via the MAP_ RESOLUTION is used in preference to computing the equivalent value from the MAP_ SCALE if both are present. AGI products assume the value to be specified in kilometers.
MINIMUM_LATITUDE / MAXIMUM_LATITUDE
The MINIMUM_LATITUDE and MAXIMUM_LATITUDE values specify the latitude extent of the data in the image and are specified in degrees. These values are not used computationally for POLAR STEREOGRAPHIC projections.
EASTERNMOST_LONGITUDE / WESTERNMOST_LONGITUDE
The EASTERNMOST_LONGITUDE and WESTERNMOST_LONGITUDE values specify the longitude extents of the data in the image and are specified in degrees. These values are not used computationally for POLAR STEREOGRAPHIC projections. See the PDS description documents for detailed information on the treatment of longitude.
LINE_PROJECTION_OFFSET
The LINE_PROJECTION_OFFSET specifies the pixel location of the center of the map projection in terms of lines from the upper left most pixel of the image.
SAMPLE_PROJECTION_OFFSET
The SAMPLE_PROJECTION_OFFSET specifies the pixel location of the center of the map projection in terms of samples along a line from the upper left most pixel of the image.
COORDINATE_SYSTEM_NAME
The COORDINATE_SYSTEM_NAME specifies the name of the coordinate system used in defining the image data. AGI products only support a value of PLANETOCENTRIC.