Monitoring Land Cover Changes in Northwestern Iran Using Training Samples Migration Method

Land cover maps are one of the fundamental parameters in geographical analysis and spatial planning. The accuracy and efficiency of land cover maps depend on three factors, including: satellite imagery, classification algorithms and training samples. The quality of the training data set has a significant impact on the accuracy of classification results. However, availability to up-to-date and high-quality training samples for land cover mapping is restricted due to the time and expense involved in preparing them. Using training sample migration techniques is one of the recently suggested alternatives. Accordingly, the primary objective of this study is to create reliable training samples using the training sample migration technique in order to track changes in the region's land cover in northwest Iran between 2002 and 2022.

The study area is located in northwestern Iran with an area of 7653 square kilometers. The study area lies between 44°35′59′′ to 46°01′25′′ longitude and 38°38′46′′ to 38°47′48′′ latitude. This study used satellite images and ground truth data.

The research methodology consists of five main steps. The initial step involved obtaining satellite images and performing pre-processing steps (radiometric and geometric correction). In the second step, training samples were collected using high-resolution satellite images (Google Earth images) and ground surveys. The third step involved the migration of training samples. To do this, the spectral similarity of the training samples from the reference and target years was first calculated using two parameters: Euclidean distance (ED) and spectral angle distance (SAD). Then, adopting the specified threshold, the migrated training samples were separated from the non-migrated samples. Furthermore, the accuracy of the migrated training samples was evaluated using reference data prepared from the Google Earth. In the fourth step, using the migrated training samples, the classification of satellite images in different years was performed. Finally, using the indices obtained from the confusion matrix, the accuracy of the classified images was evaluated.

The results showed that the threshold of 0.9 to 1.1 is the optimal threshold for separating migrated training samples from non-migrated training samples in different years. We also found a reverse correlation between the accuracy of the migrated training samples and the percentage of the migrated training samples, with higher percentages resulting in lower accuracy.

The accuracy assessment of the migrated training samples based on each parameter (SAD and ED) showed that the migrated training samples using the SAD parameter have higher accuracy than the migrated training samples using the ED parameter. Moreover, employing the migrated samples considering both parameters has increased the accuracy by 10.45% compared to using the ED parameter to migrate the training samples, and by 5% compared to using the SAD parameter to migrate the training samples.

The analysis of the percentage of migrated training samples in different land cover classes showed that, on average, 80.6% of the training samples of the water class, 75.4% of the bare land, 71.2% of the built-up, 64.6% of the grassland, 60.2% of the cropland, and 54.4% of the wetland were migrated from the reference year (2022) to each of the target years (2002, 2007, 2012, and 2017). The accuracy assessment of the migrated training samples in different land cover classes also showed that the water, built-up, bare land, grassland, cropland, and wetland classes had the highest accuracy in the migrated training samples, in that order.

The classification of satellite images was performed using Landsat images between 2002 and 2022. Accordingly, satellite images were classified into six different land cover classes. The accuracy assessment results showed that the overall accuracy of the classified images in 2022, 2017, 2012, 2007, and 2002 was 94.95%, 91.93%, 90.74%, 89.45%, and 88.94%, respectively. The accuracy assessment of different land cover classes based on two parameters, producer accuracy and user accuracy, showed that the water class has the highest producer and user accuracy among different classes (98.2% and 99.34%, respectively in 2022). In contrast, the wetland class had the lowest producer and user accuracy (90.1% and 91.25%, respectively in 2022).

The analysis of land cover changes between 2002 and 2022 showed that the area of bare land, water, and wetland classes decreased from 2002 to 2022, while the area of built-up class increased during this period. Furthermore, the grassland and cropland classes did not exhibit a constant trend of change during this time period, and their trends varied by year. However, the area of these two classes increased in 2022 compared to 2002. The analysis of the changes in the area of built-up class throughout this period shows a significant increase in the area of this land cover, which has increased from 20.38 square kilometers in 2002 to 123.98 square kilometers in 2022.

It is suggested that in future studies, other satellite images, like Sentinel-2, be used to migrate training samples in order to evaluate the effect of different spectral bands and satellite images on the migration of training samples. In addition, investigating the effectiveness of the training sample migration method in migration training samples of other land covers can be one of the research topics in future studies.