Use this procedure to calculate the geometry for HDF- and CEOS-format SeaWiFS data. This procedure extracts header information and calculates the following geometry values for each pixel.

  • Latitude and longitude
  • Sensor and solar positions

    Azimuth angle: The direction from each pixel to the sensor or sun. North equals 0 degrees, and the angles increase in a clockwise direction (for example, east equals 90).

    Zenith angle: The direction measured vertically from each pixel to the sensor or sun. Straight above the pixel equals 0 degrees.

  • Universal Coordinated Time (UTC)

ENVI Classic uses the same solar geometry calculations as the NOAA Earth System Research Laboratory (see https://gml.noaa.gov/), except that it uses lookup tables to obtain equations for time, declination, and Julian day.

Syntax

ENVI_DOIT, 'ENVI_SEAWIFS_GEOMETRY_DOIT' [, CEOS=string] [, COMP_FLAG=integer], DIMS=array, FID=file ID, /IN_MEMORY [, OUT_BNAME=string array], OUT_NAME=string [, OUT_DT={4 | 5}] [, R_FID=variable]

Keywords

CEOS (optional)

Use this keyword to specify the name of the CEOS annotation file containing the geospatial parameters. If you do not set this keyword, the file is assumed to be in HDF format.

COMP_FLAG (optional)

Set this keyword to one of the following values to indicate what geometry values are calculated.

  • 1: Latitude
  • 2: Longitude
  • 4: Sensor azimuth
  • 16: Solar azimuth
  • 32: Solar zenith
  • 64: UTC time

Set this keyword bitwise to allow multiple results by adding integer values. For example, to only calculate longitude (COMP_FLAG=2) and UTC time (COMP_FLAG=64), set COMP_FLAG to a value of 66.

If you do not set this keyword, all the geometry values are calculated by default.

DIMS

The “dimensions” keyword is a five-element array of long integers that defines the spatial subset (of a file or array) to use for processing. Nearly every time you specify the keyword FID, you must also specify the spatial subset of the corresponding file (even if the entire file, with no spatial subsetting, is to be processed).

  • DIMS[0]: A pointer to an open ROI; use only in cases where ROIs define the spatial subset. Otherwise, set to -1L.
  • DIMS[1]: The starting sample number. The first x pixel is 0.
  • DIMS[2]: The ending sample number
  • DIMS[3]: The starting line number. The first y pixel is 0.
  • DIMS[4]: The ending line number

To process an entire file (with no spatial subsetting), define DIMS as shown in the following code example. This example assumes you have already opened a file using ENVI_SELECT or ENVI_PICKFILE:

  envi_file_query, fid, dims=dims

FID

The file ID (FID) is a long-integer scalar with a value greater than 0. An invalid FID has a value of -1. The FID is provided as a named variable by any routine used to open or select a file. Often, the FID is returned from the keyword R_FID in the ENVIRasterToFID routine. Files are processed by referring to their FIDs. If you work directly with the file in IDL, the FID is not equivalent to a logical unit number (LUN).

IN_MEMORY

Set this keyword to specify that output should be stored in memory. If you do not set IN_MEMORY, output will be stored on disk and you must specify OUT_NAME (see below).

OUT_BNAME (optional)

Use this keyword to specify a string array of output band names.

OUT_DT (optional)

This keyword indicates the IDL data type of the output data. Set the keyword to one of the following integer values:

  • 1: Byte (8 bits)
  • 2: Integer (16 bits)
  • 3: Long integer (32 bits)
  • 4: Floating-point (32 bits)
  • 5: Double-precision floating-point (64 bits)
  • 6: Complex (2x32 bits)
  • 9: Double-precision complex (2x64 bits)
  • 12: Unsigned integer (16 bits)
  • 13: Unsigned long integer (32 bits)
  • 14: Long 64-bit integer
  • 15: Unsigned long 64-bit integer

You may lose precision if you specify any data type other than floating-point or double-precision floating-point.

The default data type is floating-point.

OUT_NAME

Use this keyword to specify a string with the output filename for the resulting data. If you set the keyword IN_MEMORY, you do not need to specify OUT_NAME.

R_FID (optional)

ENVI Classic library routines that result in new images also have an R_FID, or “returned FID.” This is simply a named variable containing the file ID to access the processed data. Specifying this keyword saves you the step of opening the new file from disk.

API Version

4.2