A novel-optimal monitoring index of rocky desertification based on feature space model and red edge indices that derived from sentinel-2 MSI image

Figures & data

Figure 1. Location of the study area.

Table 1. Band information of sentinel-2A and sentinel-2B.

Band	Sentinel-2A		Sentinel-2B		resolution /m
	Central wavelength /nm	Band width /nm	Central wavelength /nm	Band width /nm
Coastal aerosol band	443.9	27	442.3	45	60
Blue band	496.9	98	492.1	98	10
Green band	560.0	45	559	45	10
Red band	664.5	38	665	39	10
Red edge band 1	703.9	19	703.8	20	20
Red edge band 2	740.2	18	739.1	18	20
Red edge band 3	782.5	28	779.7	28	20
Near infrared band	835.1	145	833.0	133	10
Narrow near infrared band	864.8	33	864.0	32	20
Water vapor band	945.0	26	943.2	27	60
SWIR-Cirrus band	1373.5	75	1376.9	76	60
SWIR1 band	1613.7	143	1610.4	141	20
SWIR2 band	2202.4	242	2185.7	238	20

Figure 2. The main research flow diagram.

Figure 3. Principle of feature Space Model.

Table 2. Calculation formula of surface parameters.

Typical parameters	Formulas
MSAVI	$\text{MSAVI}=\frac{2\times {\text{B}}_{8\text{a}}+1-\sqrt{(2\times {\text{B}}_{8\text{a}}+1)-8({\text{B}}_{8\text{a}}-{\text{B}}_4)}}{2}$
NDVI	$\text{NDVI}=\frac{{\mathrm{B}}_{8\mathrm{a}}-{\mathrm{B}}_4}{{\mathrm{B}}_{8\mathrm{a}}+{\mathrm{B}}_4}$
RBI	$\text{RBI}=\frac{{\text{NDRI}-\text{NDRI}}_0}{{\text{NDRI}}_{\mathrm{r}}{-\text{NDRI}}_0},\text{NDRI}=\frac{{\mathrm{B}}_{11}-{\mathrm{B}}_{8a}}{{\mathrm{B}}_{11}+{\mathrm{B}}_{8a}}$
BSI	$\text{BSI}=(({\mathrm{B}}_{11}+{\mathrm{B}}_4)-({\mathrm{B}}_{8\mathrm{a}}+{\mathrm{B}}_2))/((({\mathrm{B}}_{11}+{\mathrm{B}}_4)+({\mathrm{B}}_{8\mathrm{a}}+{\mathrm{B}}_2))$
DI	$\text{DI}=0.0315\times {\mathrm{B}}_2+0.2021\times {\mathrm{B}}_3+0.3102\times {\mathrm{B}}_4+0.1594\times {\mathrm{B}}_{8a}-0.6806\times {\mathrm{B}}_{11}-0.6109\times {\mathrm{B}}_{12}$
Albedo	$\mathrm{Alebedo}=0.356\times {\text{B}}_5+0.13\times {\text{B}}_4+0.373\times {\text{B}}_{8\text{a}}+0.085\times {\text{B}}_{11}+0.072\times {\text{B}}_{12}-0.0018$
NDre1	$\text{NDre}1=\frac{{\mathrm{B}}_{8\mathrm{a}}{-\mathrm{B}}_5}{{\mathrm{B}}_{8\mathrm{a}}{+\mathrm{B}}_5}$
NDre2	$\text{NDre}2=\frac{{\mathrm{B}}_{8\mathrm{a}}{-\mathrm{B}}_6}{{\mathrm{B}}_{8\mathrm{a}}{+\mathrm{B}}_6}$
NDre3	$\text{NDre}3={\frac{{\mathrm{B}}_{8\mathrm{a}}{-\mathrm{B}}_7}{{\mathrm{B}}_{8\mathrm{a}}{+\mathrm{B}}_7}}_{}$

Note: B₂, B₃, B₄, B₅, B₆, B₇, B_8a, B₁₁ and B₁₂ respectively refer to the reflectance of blue, green, red, red edge band 1, red edge band 2, red edge 3, short-wave infrared 1 and short-wave infrared 2; the NDRI refers to the normalized rock index; NDRI₀ refers to the NDRI value without bare rock; NDRI_r is completely composed of bare rock NDRI value.

Figure 5. Spatial distribution of rocky desertification in BSI-RBI feature space (a) No; (b) Slight; (c) Moderate; (d) Intensive; (e) Severe.

Figure 6. Spatial distribution of rocky desertification in BSI-NDre1 feature space (a) No; (b) Slight; (c) Moderate; (d) Intensive; (e) Severe.

Figure 7. Construction of BSI-RBI feature space monitoring index (point-to-point type).

Figure 8. Construction of BSI-NDre1 feature space monitoring index (point-to-line type).

Figure 10. Validation of the inversed rocky desertification index with Google Earth images (BSI-NDre1 feature space monitoring index) (a) No; (b) Slight; (c) Moderate; (d) Intensive; (e) Severe.

Table 3. Error matrix of the inversed rocky desertification index (BSI-NDre1 feature space) and observed value.

BSI-NDre1	Inversed value
		No	slight	moderate	intensive	severe	Sum
Observed value	No	41	1	2	0	0	44
	Slight	1	55	4	0	0	60
	Moderate	2	4	69	0	0	75
	Intensive	0	0	0	53	3	56
	Severe	0	0	0	3	22	25
	Sum	44	60	75	56	25	260

Table 4. Accuracy comparisons of feature space monitoring index based on two types (P < 0.01).

Model type	Rocky desertification index	Precision (%, R^2)	RMSE
Point-to-Line	BSI-NDre1	95.1	0.872
	BSI-NDre2	91.9	1.284
	DI-NDre1	90.7	1.692
	DI-NDre2	89.5	1.981
	BSI-MSAVI	86.4	3.847
	DI-MSAVI	83.1	4.963
	BSI-NDVI	77.7	6.984
	DI-Albedo	74.7	8.347
Point-to-Point	BSI-RBI	86.5	2.343
	MSAVI-RBI	84.4	4.217
	DI-RBI	81.2	5.147
	NDVI-RBI	79.2	5.964

Figure 11. Spatial distributions of rocky desertification in Dafang District.

Data availability statement

The data that support the findings of this study are available from the corresponding author, Guo B, upon reasonable request.