What data can be stored in a GeoTIFF file?
Within Global Mapper’s extensive list of supported file formats support—300 and counting—there are several that are more commonly encountered, and one of those is the TIF format. You may know it as TIF, TIFF, or GeoTIFF, but in the end, this format holds raster data and the filenames end in a *.tif extension. So why are TIF files so common, and how can they be used in Global Mapper?
TIF, or TIFF stands for Tag Image File Format. Data in this format is pixel-based, and it is often used for high-quality graphics, making it a great fit for image data in geospatial workflows. Now that we know TIF format stores pixel-based data, what’s the deal with GeoTIFF?
GeoTIFF is a variation of the TIF format that adds a set of tags containing geospatial data in order to provide internal georeference information for the raster data in the file. This means that a single GeoTIFF can be loaded into Global Mapper and will display at the intended and geodetically correct location with no manual rectification needed. So, with the ability to hold georeferenced raster data, what does this data look like?
Single-band raster data contains a single value per pixel. This translates to an array of values that can be interpreted as colors to create an image or elevation value to build a terrain model. While there are a few ways in which a single value pixel can be interpreted and used, the most basic is a grayscale image.
Data in GeoTIFF format is stored as binary values at different scales. A grayscale GeoTIFF image can be generated in Global Mapper using 1-bit, 8-bit, 16-bit, or 32-bit values. Bit values work in binary, a base two system, so a single bit image has 21 possible values, an 8-bit image has 28, a 16-bit image has 216, and so on. Overall, the higher the bit value for the file the more possible values can be assigned to individual pixels. The most common variants are 8- or 16-bit images that contain values 0 to 255 or 0 to 65535 respectively.
A palette image also contains a single band of values, but a defined set of colors, or a palette, is used to visualize the data. This image type is used for displaying discrete area classifications, such as land cover, in raster format.
The National Land Cover Database (NLCD) is an example of a palette raster image that describes the land cover of the United States in 30m square pixels. NLCD data can be stored as a palette GeoTIFF using the defined National Land Cover Database palette of colors. The palette image is an 8-bit image providing the opportunity for up to 256 values to be used, however, only 26 values are included in the palette corresponding to different land cover types.
Another single band variation of the GeoTIFF format is elevation data. In this case, the single value per pixel is interpreted as vertical elevation and used to build and display a 3D digital elevation model.
In Global Mapper, elevation GeoTIFF files can contain 16- or 32-bit integer values, or 32-bit floating-point values. Floating point and integer designations reflect the type of data that is stored; integer values are whole numbers only while floating-point values can contain decimals allowing for partial values. This difference in the data storage method is important when working with elevation values because it impacts the units that are applied when data is exported. With an integer type, data exported using the vertical unit meters will only record whole meter intervals creating a stepped look in the exported data. A simple solution to this issue is to export the elevation data with a smaller unit, such as centimeters, to capture additional vertical detail.
While there are many ways to utilize and analyze single-band raster data, the GeoTIFF format is able to store multiple bands of data in a single file. Multiband data contains multiple values per pixel, which effectively compresses multiple single-band layers into a single layer. Combining multiple data values per pixel in a single file allows the values to be combined for data display.
Traditional true-color, RGB images consist of multiple bands containing a separate band for red, green, and blue values. These images are often 3-band, 24-bit images, with each band containing 8-bit data, but Global Mapper provides the option to create multiband images containing many more bands, each with 8-, 16- or 32-bit samples. This ability to contain many values per pixel allows bands from satellite-collected data to be stored in a single GeoTIFF file and displayed using different band combinations in Global Mapper. For more information on working with data in multiband image format, check out this blog post on Working with Satellite Data.
Proving to be a versatile format, GeoTIFF can store many types of georeferenced, pixel-based data, making it a good choice for saving and sharing raster data layers. If you are interested in working with GeoTIFF data in Global Mapper or converting this format to one of the many other raster image and elevation formats supported in Global Mapper, download a 14-day free trial today. If you have any questions, please contact us!
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