The Export GeoTIFF command allows the user to export any loaded raster, vector, and elevation data sets to a GeoTIFF format file.
When selected, the command displays the GeoTIFF Export Options dialog (pictured below) which displays export options. The dialog consists of a GeoTIFF Options panel, a Gridding panel, and an Export Bounds panel which allows the user to set up the portion of the loaded vector data they wish to export.
The File Type section allows you to choose what type of GeoTIFF file to generate. The various file types are described below:
8-bit
Palette Image - This option generates a 256-color raster GeoTIFF file
with 8-bits per pixel. The Palette options described below will apply
in this case. This option will generate a relatively small output
file, at the expense of some color fidelity depending on the palette
that you choose. The image data will be compressed using the PackBits
compression algorithm.
24-bit
RGB - This option generates a raster GeoTIFF file with 24-bits per
pixel. Uncompressed GeoTIFF images generated with this option will
be at least 3 times the size of those generated with the 8-bit Palette
option, but the colors in the image will exactly match what you see
on the screen. You can also maintain the exact colors while achieving
some compression using the LZW compression option. Selecting the JPEG
compression option generates a raster GeoTIFF file with 24-bits per
pixel, but with the raster data compressed using the JPG compression
algorithm. GeoTIFF images generated with this option will maintain
good color fidelity and often be highly compressed, although they
will lose some information as compared to the uncompressed 24-bit
RGB option. If selecting this, option keep in mind that many software packages do not yet support GeoTIFF files that
use the JPEG-in-TIFF compression option. By default the JPG compression
used in the GeoTIFF file uses a quality setting of 75, which can be
modified on the displayed options dialog.
Multi-band
- This option generates a raster GeoTIFF file with 1 or more bands
of data at either 8-, 16-, or 32-bits per band of data. This option
is very useful when working with multi-spectral imagery with more
than 3 bands of data, such as RGBI, Landsat imagery, or data sets with
more than 8 bits per color channel. After selecting this option additional dialogs will be presented
allowing for further setup of the multi-band export by choosing the
input sources for each band in the output image.
Black
and White - This option generates a two color GeoTIFF file with 1 bit
per pixel. This will generate the smallest image of these options. However if the
source image had more than two colors the resulting image will be
very poor. By default, white will be a value of 0 and black will be
a value of 1. This can be reversed by selecting the Grayscale
- Min Is Black palette option.
Elevation (16-bit integer samples) - This option generates an elevation GeoTIFF using the currently loaded elevation grid data sets. Elevation samples will be stored as signed 16-bit integers. Keep in mind there are only a handful of software packages that correctly recognize a vertical GeoTIFF.
Elevation (32-bit integer samples) - This option generates an elevation GeoTIFF using the currently loaded elevation grid data sets. Elevation samples will be stored as signed 32-bit integers. Check that vertical units are appropriate prior to using this option, intended for use with high resolution, sub-metric elevation data and values.
Elevation (32-bit floating point samples) - This option generates an elevation GeoTIFF using the currently loaded elevation grid data sets. Elevation samples will be stored as 32-bit floating point values. Keep in mind there are only a handful of software packages that can correctly recognize a vertical GeoTIFF.
Note: The Vertical
Units field allows the user to select the vertical units to use when exporting
elevation data (i.e. meters or feet). Any input data not in the selected
vertical units will be automatically converted on export.
When generating a 256 color (8-bits per pixel) GeoTIFF, it is necessary to select a palette indicates what 256 colors will be used to describe the image being exported. The following choices of palette are available:
Image
Optimized Palette - The palette generated will be an optimal mix of
up to 256 colors that will closely represent the full blend of colors
in the source images. This option will generate the best results,
but can more than double the export time required if any high color
images are present in the export set. If all of the input data is
palette-based and the combined palette of those files has 256 colors
or less, then the combined files of the input file will just be used
with no additional export time being required.
Grayscale
Palette - This palette consists of 256 scales of gray ranging from
black to white.
DRG
Optimized Palette - This palette is optimized for the exporting USGS
DRG data. The palette consists of only the standard DRG colors.
DRG/DOQ
Optimized Palette - As the name suggests, this palette is optimized
for exporting a mixture of USGS DRG data and grayscale satellite photos
(i.e. USGS DOQs). The palette consists of the 14 standard DRG colors
with the remaining 242 colors being a range of gray values ranging
from black to white.
Halftone
Palette - The palette consists of a blend of 256 colors evenly covering
the color spectrum. This palette is the best choice when exporting
anything but DRGs and grayscale satellite photos.
Custom
Palette from File - This option allows the user to choose a .pal file
describing the palette to use for the export. A .pal file should be
a text file with one line per color, with the red, green, and blue
color components for each color in the palette separated by a comma.
A .pal file for an existing palette-based file can be saved by opening
the Overlay Control Center, selecting the palette-based layer,
Options, Transparent Color button, then selecting the option
to save a color palette file.
Grayscale
- Min is Black Palette - This palette creates an 8-bit per pixel grayscale
image with no color map stored in the image. Black will be stored
as zero with varying shades of gray up to white with a value of 255.
Grayscale - Min is White Palette - This palette creates an 8-bit per pixel grayscale image with no color map stored in the image. White will be stored as zero with varying shades of gray up to black with a value of 255.
The Sample Spacing section allows the user to select the grid spacing to use when generating the file. The default value is the average of the grid spacings of all the currently loaded raster and elevation overlays.
If the Always Generate Square Pixels option is checked, the smaller of the specified x and y resolutions will be used for both the x and y resolution.
Selecting Always Generate Square Pixels ensures that the resultant image file will look good even in software that is not able to deal with pixels that aren't square.
To specify the spacing in units other than those of the currently selected view/ export projection, press the Click Here to Calculate Spacing in Other Units button.
If you want to generate a GeoTIFF file corresponding to a particular scale relative to the selected DPI value (see below), you can check the Export at the Fixed Scale option and then specify the scale to use. For example, if you specify a scale value of 25000, each inch in the output (an inch being the number of pixels equal to the specified DPI value) will be approximately equivalent to 25,000 inches on the ground.
The DPI Value to Save in Image option allows you to specify a DPI (dots per inch) value to save in the TIFF header. Some software, in particular graphics editing software, makes use of this value when sizing TIFF files for printout. Specifying the default value of 0 will result in the DPI tag not being saved to the TIFF file at all.
The Compression selection allows you to select what type of compression to use for the selected export file type. The available compression types are as follows:
No
Compression - The exported data is not compressed.
Packbits
- The palette-based image will be compressed using the lossless Packbits
algorithm.
LZW
Compression - The data will be compressed using the lossless LZW algorithm.
Note that not all applications can load a LZW-compressed GeoTIFF file.
CCITT/Fax4
- The 1-bit black-and-white image will be compressed using the lossless
CCITT/Fax4 algorithm.
JPEG Compression - The 24-bit color image will be compressed using the lossy JPEG algorithm. Note that not all applications support loading JPEG-in-TIFF encoded files.
For 8-bit palette, 24-bit RGB, or JPEG-in-TIFF files, an alpha channel will be added to the created GeoTIFF file to indicate which pixels should be treated as transparent. Note that this will create a larger file and not all applications will support TIFF files with alpha channels.
The GeoTIFF file will use a tile-based organization rather than a strip/scanline-based orientation. A tile-based orientation has advantages when zoomed in on a layer for display, but can be slower when zoomed further out. By default a tile size of 128x128 will be used, but this can be customized by creating a DWORD registry key value 'HKEY_CURRENT_USER\Software\Global Mapper\GeoTIFFExport_TileSize' with the desired tile size (such as 256 for 256x256 tiles).
The distance scale, elevation legend, and coordinate grid will be saved to the GeoTIFF file (except vertical GeoTIFFs) if they are configured to show up in the main display.
Any loaded vector data that is configured to show up in the main display will be saved to the GeoTIFF file (except vertical GeoTIFFs).
A TIFF world file will be generated with the same name as the GeoTIFF file with a .tfw extension. The TFW file is used by software that is not capable of reading the placement of the GeoTIFF file directly from the GeoTIFF header.
Any small areas with missing data will be filled in by interpolating the surrounding valid data. This is useful for filling small gaps between adjacent tiles or small holes in elevation data.
A PRJ file describing the projection of the coordinates in the file will automatically be created .
A .map file describes the location and projection of the data, similar to a PRJ or TWF file. This file is used by OziExplorer and can be read by other applications as well.
No GeoTIFF tags will be written to the generated file. This is useful for using things like datum specializations that can't be exactly stored in GeoTIFF files, when a PRJ file is preferred