Optional ENVI Header Fields

This topic describes optional metadata fields for ENVI header files.

From the Toolbox, select Raster Management > Edit ENVI Header. In the Edit ENVI Header dialog, set metadata fields as described in the following sections:

• Acquisition Time
• Associated DEM
• Auxiliary RPC Info
• Bad Bands List
• Band Names
• Class Colors
• Class Names
• Cloud Cover
• Complex Function
• Custom Metadata Item
• Data Gain Values
• Data Ignore Value
• Data Offset Values
• Data Reflectance Gain Values
• Data Reflectance Offset Values
• Default Bands to Load
• Default Stretch
• Description
• FWHM
• Geographic Corners for Non-Georeferenced Files
• Number of Classes
• Product Type
• Read Procedures
• Reflectance Scale Factor
• Security Tag
• Sensor Type
• Solar Irradiance
• Sun Azimuth
• Sun Elevation
• Wavelength
• Wavelength Units
• X Start
• Y Start
• Z Plot Ranges
• Z Plot Titles

Acquisition Time

Use Acquisition Time to denote the date and time the image was acquired. The string must comply to the ISO-8601 standard and can be in any of the following formats:

YYYY-MM-DD

YYYY-MM-DDTHH:MM:SS.DZ

YYYY-MM-DDTHH:MM:SS:Dooo:mm

Where:

• YYYY is the four-digit year
• MM is the two-digit month
• DD is the two-digit day
• T separates the date and time
• HH is the two-digit hour
• MM is the two-digit minute
• SS is the two-digit second
• D is the decimal fraction of a second with up to double-precision
• Z indicates that the time is in Coordinate Universal Time (UTC)
• ooo is a two-digit offset in hours from UTC time. If the offset is negative, a minus symbol (-) precedes the value.
• mm is an optional partial-hour offset (in minutes) from UTC time

Associated DEM

Use the Associated DEM field to associate a digital elevation model (DEM) with an image. Remove the field to end existing file associations.

Note: When you choose a DEM band, ENVI does not verify that both files are georeferenced and cover the same area. ENVI also automatically adds a geoid offset and assumes that the DEM is in meters above sea level, even for fixed-level DEMs.

1. Click the Edit button.
2. Select a DEM band.

3. Click OK.

   When you select a DEM association, two fields are written to the ENVI header file:

Field	Description
dem file = /path/file	Path and filename of selected DEM file.
dem band = 2	Index (starting at 1) of the selected DEM band. The demband is not written if the DEM file contains a single band, or if the first band of an image was chosen. In these cases, the demband value defaults to 0.
Neither field is written if an in-memory band is selected as the associated DEM band. In this case, the DEM association exists for the current ENVI session only; it does not persist for subsequent sessions.

When you open an image that has an associated DEM, ENVI adds both the image and the DEM to the Layer Manager. If ENVI cannot find the associated file, ENVI displays an error message and adds only the base image to the Layer Manager.

This DEM association also affects the following functions in ENVI.

• Cursor Location/Value Tool: The DEM value for a given pixel displays next to the data value for areas where the DEM and the image share common geographic coordinates; for example, Data:11  (DEM=1280). The DEM value is not shown if either the image or the DEM are not georeferenced.
• 3D SurfaceView: ENVI uses the associated DEM file as the default and does not prompt you to select a DEM for the surface.

• RPC or RSM Projection Emulation: If an image displays using RPC or RSM projection emulation and it has an associated DEM, ENVI uses the DEM to refine the RPC or RSM solution, which improves its positional accuracy.

  For RPC or RSM Projection Emulation, the DEM must contain map information covering the area for the RPC or RSM image; otherwise, ENVI uses the default elevation.

• RPC or RSM Orthorectification: If an image being orthorectified has an associated DEM file, then ENVI uses it as the default elevation input to the orthorectification process. You can change this default designation at any time.

Auxiliary RPC Info

Use Auxiliary RPC Info to provide geolocation information for files that have RPC reference information instead of a standard map projection.

Bad Bands List

Use the Bad Bands List to designate known-bad image bands in the image header file.

• Click the Add button to display a list of available bands for the image.
• Select the bands you know to contain invalid data and click OK.
• Remove list entries by selecting the bands from the displayed Bad Bands List, then clicking Remove selected.

Band Names

The Band Names field lists the default band names for the image and allows you to substitute names that are more meaningful for your purposes. Select the band to rename, and enter a new value in the Edit field. The Band Names list will reflect the change when you press Enter or click elsewhere.

Class Colors

The Class Colors field displays a list of assigned class colors and allows you to assign new ones more meaningful for your purposes. Double-click the class to modify; a color table displays. Select a standard ENVI or system color, or define a custom RGB combination for the selected class and click OK.

Class Names

The Class Names field lists the default class names for a classification image and allows you to substitute ones that are more meaningful for your purposes. Select the class to rename, and enter a new value in the Edit field. The Class Names list will reflect the change when you press Enter or click elsewhere.

Cloud Cover

The Cloud Cover field allows you to specify the percentage of cloud cover within the image.

Complex Function

The Complex Function field specifies the values to calculate from a complex image and to use when displaying the image, calculating statistics for the image, or writing the image to a new file.

Select the desired lookup function from the drop-down list:

• Real (real portion of number)
• Imaginary (imaginary portion)
• Power (ln(magnitude); default)
• Magnitude (square root of sum of the squares of the real and imaginary)
• Phase (arc tangent of imaginary divided by real).

Custom Metadata Item

You can add your own metadata fields to an image by using Custom Metadata Item. Field names are stored and displayed in lower case only, and special characters are not permitted. The field values have no such restrictions.

Data Gain Values

The Data Gain Values field lists the default gain values for each band in the image. You can modify these values by selecting a band and entering a new data gain in the field located below the band list. Units are W/(m² * µm * sr).

Data Ignore Value

The Data Ignore Value field allows you to specify pixel values that should be ignored in image processing workflows such as Feature Extraction, or to specify pixel values that should be transparent when displayed in the Image window.

Data Offset Values

The Data Offset Values field lists the default offset values for each band in the image. You can modify these values by selecting a band and entering a new offset in the field located below the band list.

Data Reflectance Gain Values

The Data Reflectance Gain Values field lists the default reflectance gain values for each band in the image. You can select a band and provide a new reflectance gain value in the field located below the band list.

Data Reflectance Offset Values

The Data Reflectance Offset Values field lists the default reflectance offset values for each band in the image. You can modify these values by selecting a band and entering a new reflectance offset in the field located below the band list.

Default Bands to Load

Use the Default Bands to Load field to specify which band numbers automatically load every time the file is opened. You can assign bands to RGB channels or assign a single band to define the image default as Grayscale.

Default Stretch

Use the Default Stretch drop-down list to specify what type of stretch ENVI will use when loading the image. See Stretch Types Background for more information about each stretch type. The default is No stretch.

• For % Linear stretching, enter the percentage of the data to clip (for example, 5%).
• For Linear Range stretching, enter the minimum and maximum pixel values to use in the stretch.
• For Optimized Linear stretching, enter the minimum and maximum percentages, along with the minimum and maximum adjustment percentages. See Optimized Linear Stretch for details on what these values mean.
• For Gaussian stretching, enter the number of standard deviations to use in the stretch. You can optionally enter a minimum and maximum pixel value to use in the stretch. For multi-band images, these values will apply to all of the bands. You cannot set individual minimum/maximum values for each band.
• For Equalization stretching, you can optionally enter a minimum and maximum pixel value to use in the stretch. For multi-band images, these values will apply to all of the bands. You cannot set individual minimum/maximum values for each band.
• For Square Root stretching, enter a square root factor. You can optionally enter a minimum and maximum pixel value to use in the stretch. For multi-band images, these values will apply to all of the bands. You cannot set individual minimum/maximum values for each band.
• For Logarithmic stretching, you can optionally enter a minimum and maximum pixel value to use in the stretch. For multi-band images, these values will apply to all of the bands. You cannot set individual minimum/maximum values for each band.

Description

Use the Description field to enter a string describing the image or the processing performed.

Fwhm (Full-Width Half Maximum)

Fwhm lists the full-width-half-maximum (FWHM) values of each band in an image. You can modify these values by selecting a band and entering a new FWHM in the field located below the band list.

Geographic Corners for Non-Georeferenced Files

If your file is not georeferenced but does include geographic information for the four corner points of the image, you can specify these coordinates in the ENVI header file. ENVI uses the four corner points to establish a pseudo spatial reference for the image.

Number of Classes

The Number of Classes field defines the number of classes, including unclassified regions, for classification files.

Product Type

The Product Type field describes the product type of the image. For WorldView and QuickBird products, valid values include:

• Basic Product
• Stereo Product
• Standard Product
• Orthorectified Product
• Digital Elevation Model Product

For NPP VIIRS products, valid values include:

• SDR
• EDR

Read Procedures

Use the Read Procedures field to specify spatial and spectral read routines in ENVI Classic custom file readers.

Reflectance Scale Factor

Use Reflectance Scale Factor to enter a scale factor that is used in ENVI’s Endmember Collection dialog to correctly scale library data or other reflectance data to match the image data. If one of the files used in the Endmember Collection dialog does not have a reflectance scale factor defined, then no scaling is applied.

1. Enter the value that, when divided into your data, would scale it from 0 to 1. For example, if the value of 10,000 in your data represents a reflectance value of 1.0, enter a reflectance scale factor of 10,000.
2. Click OK.

Security Tag

Include the Security Tag field to provide a string with security information inherited from formats that typically contain security classification levels (such as NITF).

Sensor Type

Select an option from the Sensor Type drop-down list to indicate the sensor used to capture the image, such as QuickBird, Landsat TM, SPOT, or RADARSAT.

Solar Irradiance

The Solar Irradiance field lists each band in the image with its corresponding top-of-atmosphere solar irradiance value. Select a band and enter a new solar irradiance value in the field located below the band list. Units are W/(m² * µm).

Sun Azimuth

Use Sun Azimuth to specify the angle of the sun (in degrees) from due north in a clockwise direction.

Sun Elevation

Use Sun Elevation to specify the angle of the sun (in degrees) above the horizon.

Wavelength

The Wavelength field lists the center wavelength values of each band in an image. Units should be the same as those used for the fwhm field and set in the wavelength units parameter. To assign a new center wavelength, select the band to be modified and enter a new value in the Edit field.

Wavelength Units

Select the Wavelength Units for all bands in the image by selecting an option from the following list:

• Micrometers
• Nanometers
• Wavenumber
• GHz
• MHz
• Index
• Unknown

X Start

Use X Start in conjunction with Y Start to define the upper-left pixel in the image. Images that are spatial subsets of larger images often use an image coordinate system that references the parent (or larger) image so that you can link and dynamically overlay the two images. The default values for the upper-left pixel are (0,0).

Y Start

Use Y Start in conjunction with X Start to define the upper-left pixel in the image. Images that are spatial subsets of larger images often use an image coordinate system that references the parent (or larger) image so that you can link and dynamically overlay the two images. The default values for the upper-left pixel are (0,0).

Z Plot Ranges

Include Z Plot Ranges metadata to specify an array that defines the default minimum and maximum values for Z plots.

Z Plot Titles

Use Z Plot Titles to specify text to display in the x and/or y axis titles for Z plots.

See Also

Edit Raster Metadata, ENVI Header Files, View Metadata