Case Study 6: A SPOT Survey of Wild Rice in Northern Minnesota

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a. Introduction.

(1) A vegetation survey of natural wild rice surrounding three neighboring lakes 200

miles (518 km) south of St. Paul, Minnesota, was conducted to provide a base map for pol-

lutant and water level monitoring. The study presented here utilized standard supervised

classification, based on ground-truth, of high-resolution SPOT data. Wild rice is a natural

marsh grass that is sensitive to water level changes and to changes in phosphorous concen-

trations; increases in phosphorous and water levels can significantly destroy wild rice com-

munities. This is of concern as this important grass is a staple in the Chippewa Indian diet

and is consumed by migratory birds.

(2) The researchers in this study were tasked with mapping and estimating the acreage

of wild rice surrounding three lakes in Minnesota. Three spectral classes where developed

with the use of a supervised classification to delineate the grass and its varying substrate.

b. Description of Methods. Ground truth data were collected simultaneously with SPOT

over flight. The ground truth data included information regarding vegetation and substrate

type as well as the sites corresponding UTM (global position in the Universal Transverse

Mercator coordinate system).

c. Field Work. In the field, 18 ground control points (GCPs) were collected for rectifi-

cation of the SPOT image and an additional 132 ground truth points were collected for the

supervised classification algorithm. This data collection coincided with the SPOT over

flight.

d. Sensor System. SPOT was chosen for its optimal detection of vegetation in the pres-

ence of inorganic ground cover (i.e., water). Vegetation absorbs both red and blue radiation,

while reflecting green and near infrared (NIR) because of chlorophyll production. This

matched well with the spectrum data provided by SPOT (which maintains green, red, and

NIR bands among others).

e. Study Results.

(1) Prior to the classification process, it had been predicted that the wild rice would

dominate one spectral class, as wild rice is spectrally distinct from other vegetation. Open-

ings in the grass canopy, however, contributed to the spectral mixing observed in the image

scene. Three spectrally distinct populations were noted, likely because of the heterogeneity

of the background reflectance, varying crop canopy, and varying water content in the sub-

strate.

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