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Frontiers of Earth Science

ISSN 2095-0195

ISSN 2095-0209(Online)

CN 11-5982/P

Postal Subscription Code 80-963

2018 Impact Factor: 1.205

Front. Earth Sci.    2021, Vol. 15 Issue (1) : 54-69    https://doi.org/10.1007/s11707-020-0861-x
RESEARCH ARTICLE
Machine learning-based crop recognition from aerial remote sensing imagery
Yanqin TIAN1, Chenghai YANG2(), Wenjiang HUANG1, Jia TANG1, Xingrong LI1, Qing ZHANG1()
1. Key Laboratory of Digital Earth Science, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100094, China.
2. USDA-Agricultural Research Service, Aerial Application Technology Research Unit, College Station, TX 77845, USA.
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Abstract

Timely and accurate acquisition of crop distribution and planting area information is important for making agricultural planning and management decisions. This study employed aerial imagery as a data source and machine learning as a classification tool to statically and dynamically identify crops over an agricultural cropping area. Comparative analysis of pixel-based and object-based classifications was performed and classification results were further refined based on three types of object features (layer spectral, geometry, and texture). Static recognition using layer spectral features had the highest accuracy of 75.4% in object-based classification, and dynamic recognition had the highest accuracy of 88.0% in object-based classification based on layer spectral and geometry features. Dynamic identification could not only attenuate the effects of variations on planting dates and plant growth conditions on the results, but also amplify the differences between different features. Object-based classification produced better results than pixel-based classification, and the three feature sets (layer spectral alone, layer spectral and geometry, and all three) resulted in only small differences in accuracy in object-based classification. Dynamic recognition combined with object-based classification using layer spectral and geometry features could effectively improve crop classification accuracy with high resolution aerial imagery. The methodologies and results from this study should provide practical guidance for crop identification and other agricultural mapping applications.

Keywords machine learning      crop recognition      aerial imagery      dynamic recognition      static recognition     
Corresponding Author(s): Chenghai YANG,Qing ZHANG   
Online First Date: 30 March 2021    Issue Date: 19 April 2021
 Cite this article:   
Yanqin TIAN,Chenghai YANG,Wenjiang HUANG, et al. Machine learning-based crop recognition from aerial remote sensing imagery[J]. Front. Earth Sci., 2021, 15(1): 54-69.
 URL:  
https://academic.hep.com.cn/fesci/EN/10.1007/s11707-020-0861-x
https://academic.hep.com.cn/fesci/EN/Y2021/V15/I1/54
Fig.1  Location of the study area: (a) airborne RGB mosaic image of July 2017, (b) RGB image of study area, (c) calibration tarps, and (d) ground control point (GCP) measurement with GPS.
Fig.2  Airborne images for this study: (a) RGB image of June, (b) Color-infrared (CIR) composite of June, (c) NDVI image of June, (d) RGB image of July, (e) CIR composite of July, and (f) NDVI image of July.
Fig.3  Flowchart of the classification methods.
Fig.4  Global image segmentation results: (a) four-band image segmentation with 449 image objects and (b) six-band image segmentation with 370 image objects.
Fig.5  Image object merged results: (a) four-band image with 277 image objects and (b) six-band image with 227 image objects.
source feature type feature name number
reference VI Difference Vegetation Index (DVI) = NIR – R (Richardson and Everitt, 1992) 13
Ratio Vegetation Index (RVI) = NI /R (Jordan, 1969)
Normalized Difference Vegetation index (NDVI) = (NIR – R) / (NIR+ R) (Rouse et al., 1974)
D_NDVI= Mean 7_NDVI – Mean 6_NDVI
Bare Index (BI) = Blue+ Red
Soil Adjusted Vegetation Index (SAVI) = 1.5 (NIR – R) / (NIR+ R+ 0.5) (Huete, 1988)
Optimization of Soil-adjusted Vegetation Index (OSAVI) = (NIR – R) / (NIR+ R+ 0.16) (Rondeaux et al., 1996)
B* = B/(B+ G+ R), G* = G/(B+ G+ R), R* = R/(B+ G+ R)
Excess Green (EG) = 2G* – R* – B* (Woebbecke et al., 1995)
Excess Red (ER) = 1.4R* – G* (Meyer et al., 1999)
VI* = EG – ER (Camargo Neto, 2004)
eCognition layer Mean of B, G, R, NIR, 6_NDVI, 7_NDVI and Brightness \Standard deviation (StdDev) of B, G, R, NIR, 6_NDVI and 7_NDVI
HIS (Hue, Intensity, Saturation)
16
geometry Area, Border length, Length/Width
Asymmetry, Compactness, Density, Shape Index
7
texture GLMC (Homogeneity, Contrast, Dissimilarity, Entropy, Angular Second Moment, Mean, StdDev, Correlation) 8
total number of features 44
Tab.1  List of object features for classification modeling
class type count percentage ?class type count percentage
bare soil and fallow 63 12.6% ?impervious 8 1.6%
corn 186 37.2% ?sorghum 34 6.8%
cotton 77 15.4% ?soybean 63 12.6%
grass 69 13.8%
Tab.2  Count and percentage by class type for 500 reference points
Fig.6  Feature optimization results of (a) four-band image and (b) six-band image based on three sets of object features: layer spectral features (L), layer spectral features and geometry features (LG), and layer spectral features, geometry features and texture features (LGT).
image feature type feature name number
four-band L DVI, RVI, NDVI, BI, SAVI, OSAVI, B*, G*, EG, ER, R*, VI*; Mean of B, G, R, NIR and Brightness; Standard deviation (StdDev) of B, G, R, NIR; HIS (Intensity, Saturation) 23
LG DVI, RVI, NDVI, SAVI, OSAVI, B*, G*, EG, ER, R*, VI*; Mean of B, G, R, NIR and Brightness; StdDev of B, G, R, NIR; HIS (Intensity)
Area, Border length, Asymmetry, Density, Shape Index
26
LGT DVI, RVI, B*, G*, EG, ER, R*, VI*; Mean of NIR; StdDev of B, G, R, NIR; HIS (Intensity)
Area, Border length, Asymmetry, Density
GLMC (Homogeneity, Dissimilarity, Entropy, Angular Second Moment, StdDev, Correlation)
24
six-band L DVI, D_NDVI, B*, G*, EG, ER, R*, VI*; Mean of G, NIR, 6_NDVI and Brightness; StdDev of B, G, NIR, 6_NDVI and 7_NDVI; HIS (Intensity) 18
LG DVI, D_NDVI, B*, G*, EG, ER, R*, VI*; Mean of G, NIR, 6_NDVI and Brightness; StdDev of B, G, NIR, 6_NDVI; HIS (Intensity)
Area, Asymmetry, Density
20
LGT D_NDVI, B*, G*, EG, ER, R*, VI*; Mean of NIR, 6_NDVI; StdDev of B, G, R, NIR, 6_NDVI and 7_NDVI; HIS (Intensity)
GLMC (Homogeneity, Contrast, Dissimilarity, Entropy, Angular Second Moment, StdDev, Correlation)
23
Tab.3  Best object features for different classification scenarios
Fig.7  Image classification results: 4= four-band image, 6= six-band image, PB= pixel-based classification, OB-L= object-based classification of the layer spectral features, OB-LG= object-based classification of the layer spectral and geometry features, and OB-LGT= object-based classification of the layer spectral, geometry and texture features.
Fig.8  Results of overall kappa and overall accuracy analysis by four classification methods: OKP = Overall Kappa, OA= Overall accuracy: 4= four-band image, 6= six-band image, PB= pixel-based classification, OB_L= object-based classification of the layer spectral features, OB_LG= object-based classification of the layer spectral and geometry features, and OB_LGT= object-based classification of the layer spectral, geometry and texture features.
classification BF CO CT GR IM SG SB
(a) 4PB Pa (%) 50.8 73.7 29.9 23.2 100 32.4 39.7
Ua (%) 57.1 69.9 63.9 35.6 80.0 14.7 30.5
Kp 0.45 0.57 0.24 0.16 1 0.20 0.28
(b) 6PB Pa (%) 79.4 88.2 32.5 20.3 100 52.9 50.8
Ua (%) 72.5 86.3 46.3 46.7 88.9 22.2 47.8
Kp 0.75 0.81 0.24 0.15 1 0.44 0.43
(c) 4OB-L Pa (%) 74.6 86.6 84.4 24.6 100 82.4 81.0
Ua (%) 92.2 91.0 76.5 68.0 100 36.8 65.4
Kp 0.72 0.79 0.81 0.21 1 0.79 0.77
(d) 6OB-L Pa (%) 95.2 97.8 68.8 53.6 100 94.7 98.4
Ua (%) 1 94.8 84.1 63.8 100 51.2 81.6
Kp 0.95 0.97 0.64 0.48 1 0.61 0.98
(e) 4OB-LG Pa (%) 82.5 60.2 67.5 63.8 100 82.4 90.5
Ua (%) 61.2 1 82.5 63.8 100 43.8 57.6
Kp 0.79 0.49 0.63 0.58 1 0.80 0.88
(f) 6OB-LG Pa (%) 95.2 96.8 89.6 49.3 100 79.4 98.4
Ua (%) 1 94.7 85.2 89.5 100 57.4 81.6
Kp 0.95 0.95 0.88 0.45 1 0.77 0.98
(g) 4OB-LGT Pa (%) 84.1 60.2 67.5 62.3 100 76.5 92.1
Ua (%) 60.9 1 89.7 62.3 100 46.4 52.7
Kp 0.81 0.49 0.63 0.56 1 0.74 0.90
(h) 6OB-LGT Pa (%) 95.2 96.8 72.7 60.9 100 82.4 84.1
Ua (%) 1 95.7 71.8 79.2 100 51.9 89.8
Kp 0.95 0.95 0.68 0.56 1 0.80 0.82
Tab.4  Accuracy assessment results by class for four-band and six-band images based on four classification methods
Fig.9  Analysis of the importance of classification methods by class: BF= bare soil and fallow, CO= corn, CT= cotton, GR= grass, IM= impervious, SG= sorghum, SB= soybean, AKp1= the average Kappa coefficient of the two data sources for each class under four classification methods, PB= pixel-based classification, OB_L= object-based classification of the layer spectral features, OB_LG= object-based classification of the layer spectral and geometry features, and OB_LGT= object-based classification of the layer spectral, geometry and texture features.
PB OB-L OB-LG OB-LGT
AOKp 0.445 0.750 0.745 0.725
AOA (%) 56.3 80.1 79.3 77.9
Tab.5  Analysis of the importance of classification methods on the overall scale
Fig.10  Analysis of the importance of dynamic recognition by class: BF= bare soil and fallow, CO= corn, CT= cotton, GR= grass, IM= impervious, SG= sorghum, SB= soybean, and AKp2= the average Kappa coefficient of the four classification methods for each class under the two recognition methods.
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