Active Layer Thickness - Alaska LandCarbon Project

The active layer thickness (ALT) map indicates the average thickness of the active layer (in centimeters) from the surface to the top of a layer of permafrost within a soil map unit. This map should be interpreted carefully based on the method of calculation. A soil horizon was flagged as having permafrost if the texture modifier for the horizon contained the characters “PF-“, indicating permafrost. A soil component without permafrost was coded as having an active layer thickness of 101 cm. This value was arbitrarily assigned so that all components would have a value to contribute to the weighted average of ALT at the map unit level. Therefore, an ALT value of 101 cm indicates that there was no permafrost within the top 100 cm for any of the soil components. Smaller numbers indicate that some permafrost was present in the soil. The numbers closer to the minimum for this data set (39 cm) indicate that a considerable part of the area had permafrost near the surface, whereas values closer to 100 indicate that only a small proportion of the area had permafrost and/or that it was close to the 100 cm depth. Additional datasets would need to be used in conjunction with this map to separate the influences of the contributions of the thickness of the active layer and the proportion of the land with permafrost in the 0-100 cm zone. A non-soil component, such as water, ice, or rock at the surface, will contribute the value 101 cm to the weighted average, and thereby represents a “not permafrost” condition, because for this database only the permafrost in soil is considered. The prefix “mu_” on the filename indicates that a map unit level variable was being computed. The “_default_101” in the filename indicates that the default value for components without permafrost in the top 100 cm was 101 cm. The suffix “_2” on the filename is an arbitrary version number. The file extension “.img” indicates an Erdas Imagine format for spatial data, which can be manipulated by various types of geospatial analysis software, including GDAL (the Geospatial Data Abstraction Library), Erdas Imagine, and ArcGIS.

For more information on the structure of soil data attributes, including definitions of map units, components, and horizons, see (which redirects to

The source of the soil data for Alaska for this project was a revised STATSGO database for Alaska at 1:500,000 scale, dated October 16, 2012. The data are in an Albers Conical Equal Area projection with a central meridian of 154° W and standard parallels at 55° N and 65° N. The cell size is 1000x1000 meter (1-km resolution), with 2012 rows and 3685 columns. The minimum value is 39.38 cm and the maximum value is 101 cm.

Data and Resources