North American Carbon Program

Sub-pixel Analysis of a 1-km Resolution Land-Water Mask

Source of Data:

The North American sub-pixel water mask product is derived from three data sets for three geographic extents:

North America (South of 60°N) – From the 1 arc-second (~30-m) resolution Shuttle Radar Topography Mission (SRTM) water body data set, the number of 1 arc-second pixels containing water within a 30 arc-second pixel were calculated to derive the sub pixel percentage of water (http://edc.usgs.gov/products/elevation/swbd.html).

Alaska (North of 60°N) – The National Land Cover Dataset (NLCD) 2001 is a land-cover classification scheme based on Landsat satellite data. This dataset is at a 30-m resolution and includes a water category. The water pixels from this land-cover data were used as input for the sub-pixel analysis (http://www.mrlc.gov/).

Canada (North of 60°N) – Hydrographic features from the National Topographic Data Base (NTDB) Maps were converted to 1 arc-second pixels, from which the number of 1 arc-second pixels containing water within a 30 arc-second pixel were calculated to derive the sub-pixel percentage of water (http://www.geogratis.ca/geogratis/en/collection/detail.do?id=8147).

Introduction:

• The North American Carbon Program (NACP) is designed to quantify the magnitudes and distributions of carbon sources and sinks, explain the processes controlling them, and produce a consistent analysis of North America's carbon budget. The Modeling and Synthesis Thematic Data Center (MAST-DC) supports NACP by providing data products and data management services needed for modeling and synthesis activities.

• In many NACP or other project’s modeling activities, it is often necessary to mask or exclude regions in water. A land-water mask is a data layer that delineates regions in land and in water and is useful for this purpose.

• On a continental and global scale, a MODIS data based land-water mask is widely used and is one of the only products available for the whole of North America. This MODIS based land-water mask is at a resolution of 30 arc-seconds or approximately 1-km and delineates water features such as oceans, shoreline, lakes etc. A 250-m MODIS land-water mask is being developed, but may not be available for several years.

• Higher resolution water layers are available for North America, but the data are from different sources and hence are at different levels of accuracy (spatial and temporal).

• However, to take advantage of the fine resolution water data, we developed a 30 arc-second resolution map that captures sub-pixel information about land and water areas. The map contains the percentage of water within each 30 x 30- arc second pixel.

• This sub-pixel product will be useful as a water mask, and will also serve to understand the 30 arc-second MODIS based land-water mask product.

Comparison of MODIS based Land-water mask and Sub-pixel Land-water mask for the Mid Continent Intensive (MCI) Region:

Within shallow Inland Water (Inland Water <5-km from shore OR <50m deep),• % containing more than 75% water = 74.5%• % containing 50 - 75% water = 10.5%• % containing 25 - 50% water = 6.9 %• % containing less than 25% water = 8.1%Within Ocean Coastlines and Lake Shorelines (LAST land pixel)• % containing more than 75% water = 8.7%• % containing 50 - 75% water = 8.8%• % containing 25 - 50% water = 13.1 %• % containing less than 25% water = 69.4%Within Land• % containing more than 75% water = 0.2%• % containing 50 - 75% water = 0.4%• % containing 25 - 50% water = 0.9 %• % containing less than 25% water = 98.5%

Environmental Sciences DivisionOak Ridge National Laboratory

Oak Ridge, TN 37830

*Latha Baskaran, Bob Cook and Suresh Kumar Santhana-Vannan

The Shuttle Radar Topography Mission (SRTM) obtained elevation data on a near-global scale to generate a high-resolution digital topographic database of Earth (February of 2000). The SRTM water body dataset was produced from 1 arc-second digital terrain elevation data by the National Geospatial-Intelligence Agency (NGA).

The National Topographic Data Base (NTDB) comprises of digital vector data sets that cover the entire Canadian landmass. The NTDB includes a layer for hydrographic features.

Canada (North of 60°N)

United States and Canada (South of 60°N)

Alaska (North of 60°N)

The National Land Cover Dataset is a Landsat based land cover database prepared by a consortium of agencies called Multi-Resolution Land Characteristics (MRLC) Consortium. The classification scheme includes a ‘water’ category which includes water bodies and generally has less than 25% cover of vegetation or soil.

Methodology:

The 1 arc-second pixels were coded with a value of 1 (water). All other regions were coded with a value of 0 (land).The ‘Block statistics’ tool from ArcTools was used to sum the number of 1 arc-second (~30-m) pixels within a non-overlapping 30 arc-second boundary. The resulting values were resampled to 30 arc-seconds and divided by 9 to get the percentage of water pixels within a 30 arc-second extent.

1 arc-second land- water mask

30 arc-second sub pixel analysis

- Water- Land

83

75

92

87

95

64

99

59

100

100

100

100 100

100

100

100

10083%

17%

(750 pixels)

150 (pixels)

Acknowledgements:

This poster is based upon work supported by the National Aeronautics and Space Administration under Agreement No. NNH06AE47I issued through the Terrestrial Ecology Program, Earth Sciences Directorate.

Sub-pixel Land-Water Mask

Contact Information:

*Latha Baskaran (baskaranl@ornl.gov)

The sub-pixel product is available from the MAST-DC website - http://nacp.ornl.gov/mast-dc/