Geoscience Reference
In-Depth Information
The total number of validation pixels to be used to assess the accuracy for each cover type was
then calculated by the general formula:
V
=
S
¥
P
(6.2)
c
t
c
where
V
= the total number of validation pixels to be used for a specific cover type,
S
= the total
c
t
number of validation pixels to be sampled for use in accuracy assessment, and
P
= the proportion
c
of the classified map predicted to be that cover type.
To illustrate this proportional sampling accuracy method, we describe the forest cover type for
the 1999 map. The cover type with the smallest number of validation pixels in 1999 was the bare
soil cover type with a total of 79 validation pixels (
). Of the total number of pixels in the 1999
classified map (8,970,395), the bare soil cover type was predicted to be 201,267 pixels, or a
proportion of 0.0224 of the total classified map (
N
s
). Using Equation 6.1 above, the resulting sample
size of validation pixels to be used for accuracy assessment of the 1999 LC map (
P
s
) was 3,521
pixels. In the 1999 map, the forest cover type was predicted to cover 68.6% of the classified map
(i.e., 6,155,275 pixels out of 8,970,395 total pixels). Using Equation 6.2 above, the sample size of
validation pixels to be used for the forest cover type (
S
t
0.686).
Once the validation sample sizes were chosen for each cover type, a standard accuracy assess-
ment was performed whereby the cover type of each of the validation pixels was compared with
the corresponding cover type on the classified map. Agreement and disagreement of the validation
data set pixels with the pixels on the classified map were calculated in the form of an error matrix
wherein the producer's, user's, and overall accuracy were evaluated.
V
) was then 2,414 (i.e. 3,521
¥
c
6.3 RESULTS AND DISCUSSION
6.3.1
Classified Imagery and Land-Cover Change
Presentation and discussion of accuracy assessment results will focus only on the 1994 and
1999 LC maps. (The 1986 map was not directly comparable because it was based on coarser
resolution and resampled MSS data and because it contained cirrus cloud cover over parts of the
study.) A visual comparison of 1986-1999 LC maps shows significant change. Plate 6.1 presents
the classified imagery with parcel boundaries overlaid for a portion of the study area near one of
the major feeder roads. In 1986, approximately 2 years after migrant settlement, only some initial
clearing was observed near roads; however, 13 years later (1999) there were significant open areas
and only a small number of parcels that remained mostly covered with primary forest. The extensive
deforestation illustrated in Plate 6.1 is confirmed by the numeric data presented in Table 6.3. In
1994, 147,380 ha, or 68.5% of the total study area (215,000 ha), was covered in primary forest.
Table 6.3
Land-Cover Change in Study Area, Rondonia 1994-1999
Area (ha)
Percentage of Area
Change in Area
1994-1999 (ha)
Percentage of
Change 1994-1999
Class
1994
1999
1994
1999
Forest
147,380
117,573
-29,806
68.5
54.6
-20.2
Secondary forest
27,759
30,732
2,973
12.9
14.3
10.7
Transition
2,234
5,555
3,321
1.0
2.6
148.6
Crops
12,072
27,833
15,760
5.6
12.9
130.5
Pasture
16,253
22,386
6,133
7.6
10.4
37.7
Bare soil
5,183
6,823
1,640
2.4
3.2
31.6
Water
4,251
4,252
1
2.0
2.0
0.0
Total
215,132
215,154
100.0
100.0
