Imagery Sources

Digital Northern Great Plains provides downloadable satellite and airborne imagery, NDVI products, and other data products as detailed below. To download these products, go to Digital NGP »

IMAGERY

Original Image

Current downloadable through Digital NGP are satellite and airborne imagery as follows:

Landsat series; band 1 - band 7
ASTER; green, red, and NIR
MODIS; red and NIR
AEROCam; green, red, and NIR

When Original Image is selected, what is delivered are image composites of up to 3 bands, as we received them but with UMAC-applied corrections (e.g., atmospheric, geo, etc.), and sub-setted for the geographical map area currently in view. See DNGP Data Sources and Image Processing for more detail on imagery sources and UMAC-applied corrections. (All of the sample products shown here are derived from raw imagery acquired by the Landsat TM sensor on August 30, 2005, of the Grand Forks ND - East Grand Forks MN local area.)

Landsat TM False color composite image.

User Selectable Band Combinations

When using DNGP, you can specify which sensor spectral bands you want, and which of them you want assigned to drive which colors on your screen. For example, in order to highlights vegetated area, imagery data can be assigned to blue, green, and red screen colors in the order of NIR(band 3), red (band 2), and green (band 1), respectively. This is a standard band and color combination known as False Color Composite. Active vegetation areas show red and pink. Waterbodies shows black and urban structures shows bright blue and gray. With this color composite scheme, it may be possible draw a distinction between different kinds of vegetation and forest. Many different band and color combinations can be selected.

Landsat TM natural color image.

Natural Color

When the Natural Color option is selected, DNGP forces the band and color combination so that the spectral band closest to the blue portion of visible light drives the blue display color; similarly green band drives green color, and red band drives red color. Natural color image format produces an image that would be very close to what we would see with our own eyes. Vegetated areas in image are represented in green undertones, black areas are waterbodies. The white and pink areas in an image typically represent soils and/or urban regions.

Unfortunately, there is no blue band available for MODIS, ASTER, and AEROCam data. For these we use our in-house products algorithm, which falsely assigns a color into the empty band and perform natural color enhancement on the image using ERDAS Imagine software. Then, we save the resulting image as a geotiff format and upload to Digital NGP as browse image.

NDVI PRODUCTS

Standard "Red" NDVI

The Normalized Difference Vegetation Index (NDVI) is an index that provides a standardized method of comparing vegetation greenness between satellite images. The formula to calculate NDVI is:

NDVI = (NIR - RED) / (NIR + RED)

The NDVI values can range from -1.0 to 1.0, but vegetation values typically range between 0.2 and 0.8. Higher index values are associated with higher levels of healthy vegetation cover, whereas clouds and snow will cause index values near zero, making it appear that the vegetation is less green. NDVI can be used as an indicator of relative biomass and greenness. If sufficient ground data is available, the NDVI can be used to calculate and predict primary production, dominant species, and grazing impact and stocking rates.

Green NDVI

The index for chlorophyll estimation should be invariant with respect to pigments other than Chlorophyll, and should not be influenced by other factors. A study showed maximum sensitivity takes place from 520 nm to 630 nm and near 700 nm. The lowest variation of reflectance took place in the near-infrared (above 750 nm) and in the blue (shorter than 500nm) parts of the spectrum. The green NDVI (or GNDVI) is calculated as:

GNDVI = (NIR - GREEN) / (NIR + GREEN)

Some studies have found the green NDVI is much more sensitive to the Chlorophyll concentration across a wide range of Chlorophyll values than the original "red" NDVI, and enabled precise estimation of pigment concentration.

PRODUCTS IN TEXT FORMAT

Precision Farming Support

Both imagery and NDVI data products are typically delivered in a raster format, specifically geotiff. To better support import of imagery into precision farming software tools, UMAC has developed the option to have these products delivered in text format. To produce this format, the latitude and longitude of the center point of each pixel is computed, and the data value of that location, along with the latitude and longitude, are provided in a text file in comma delimited rows.