Geoscience Reference
In-Depth Information
only some of the mapped area, or (2) allow samples to be chosen randomly, resulting in some
samples landing in areas where existing NTM was not immediately available for reference data
development. The latter approach was selected because limiting the accuracy assessment area was
considered statistically unacceptable. To overcome the NTM data gaps, first-stage samples were
chosen prior to receipt of the final map. This provided additional time for the acquisition of new
NTM data. Persistent data gaps were supplemented by the interpretation of TM composite images.
First stage sample units were 15-min quadrangle areas. To ensure that an adequate number of
accuracy assessment sites per cover class were sampled, quadrangles were selected for inclusion
in accuracy assessment based on the diversity and number of cover classes in the quadrangle. A
relative diversity index was determined through the screening of TM composite images of the study
area. The number and diversity of cover type polygons were summarized for each quadrangle, and
the six quadrangles with the greatest cover type diversity and largest number of classes were selected
as the first-stage samples.
The second-stage sample units were the polygons of the LC map vector file. Fifty polygons
per class were randomly selected across all the six quadrangles. If fewer than 50 polygons of a
particular class existed within the six quadrangles, then all the available polygons in that class were
selected. Both primary and secondary sample selection was automated using accuracy assessment
software developed for this project.
12.3.3
Site Labeling
All accuracy assessment samples had two class labels: a map label and a reference site label.
For this project, the “map” label was automatically derived from the LC polygon map label provided
by Earthsat and stored for later use in the compilation of the error matrix. An expert analyst, based
on image interpretation of NTM data, manually assigned the corresponding “reference” label. Each
sample polygon was automatically displayed on the computer screen simultaneously with the
assessment data form (Figure 12.1). The analyst entered the label for the site into the form using
the imagery and other ancillary data available. To ensure independence, at no time did the image
analyst labeling the samples have access to map data.
To account for variation in interpretation, the accuracy assessment analyst also completed a
LC-type fuzzy logic matrix for every accuracy assessment site (Figure 12.1). Each polygon was
evaluated for the likelihood of being identified as each of the possible cover types. First, the analyst
Figure 12.1
Form for labeling accuracy assessment reference sites.
