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A land mask tells which parts of the Earth are covered by land (as opposed to water). It is a tool that can be used to obtain information separately for land and water areas of the Earth. For example, one might want to calculate how much snow is on a continent, without including the surrounding ocean areas. Or, one might want to separately calculate the percent cloud coverage over land and over water.

A land mask is made from a static map using mean water level. It does not account for the effect of tides and other changes in water levels.

Resolution

A land mask is constructed from high resolution information about the Earth’s surface. For illustration, the picture at left shows a 1 by 1 degree latitude longitude grid box that contains the site of NASA Langley Research Center. It includes land, ocean, the Chesapeake Bay, and a number of rivers. This high resolution imagery (from Google Earth) is mainly from aerial photography.

One degree of latitude or longitude is divided into 60 minutes of arc (denoted 60′). The very small dark-colored box in the center of the image is a 1′ by 1′ lat/long grid box. It takes 60 times 60, or 3600, of these to fill a 1 degree lat/long grid box.

Since this kind of high resolution aerial imagery is not available for the whole planet, we start our land mask from a 10′ water percentage map. The larger, yellowish box in the middle of the image is a 10′ by 10′ lat/long grid box (1/6 by 1/6 degree of lat/long). At the Equator, that corresponds to about 18 by 18 square kilometers. For the 10′ grid box in the image at left, the water percentage is about 94% (it is a little hard to tell from the image because of the way that the aerial photographs have been pieced together).

The high resolution water percentage map is then averaged to the resolution of interest for a particular purpose. Within the MY NASA DATA LAS we offer a 1 degree and a 2.5 degree resolution land mask. The 1 degree land mask has a single value for the image at left: land, water or coast. This particular box would be identified as coast because it is a mixture of land and water.

Threshold

While some grid boxes are entirely filled with land, and others are entirely filled with ocean, there are of course some places – like the image above – where grid boxes contain both land and water. A decision must be made about the threshold amount of water that is allowed in a grid box that is identified as land (and about the amount of land that is allowed in a grid box that is identified as water). The maps at right show what a land (white), ocean (black), coast (grey) land mask looks like for a very restrictive threshold (1%) to a very inclusive threshold (20%).

In the restrictive threshold, any grid box that has between 1 and 99 % water is designated as coast. This results in a large number of grid boxes designated as coast, especially around island archipelagos.

In the inclusive threshold, a grid box can contain up to 20% water and still be designated as land. Similarly, a grid box can contain up to 20% land and still be designated as water.

For MY NASA DATA, we have selected a 10% threshold for the land mask. This is a reasonable intermediate value that captures most of the land area without an excessive amount of coast.

Click on image to view larger pdf image.

Values

A land mask assigns a code to identify each latitude/longitude grid box. In MY NASA DATA, we use a value of 0 for water, 1 for land, and 2 for grid boxes that contain both land and water. These boxes are designated as coast, although they also occur around inland lakes and rivers.

Grid boxes that are designated as coast will change depending on the averaging resolution that is used. The two images at the left show the 10% threshold land masks at the 1 and 2.5 degree latitude/longitude resolutions used in MY NASA DATA. Since the 2.5 degree grid boxes are larger, more of them are designatedas coast because more have a mixture of land and water.