[
  [[60, 24], [78, 24], [78, 37], [60, 37], [60, 24]],   # CPEC (China-Pakistan Economic Corridor)
  [[88, 20], [102, 20], [102, 28], [88, 28], [88, 20]], # BCIM Corridor
  [[95, -8], [122, -8], [122, 10], [95, 10], [95, -8]], # Maritime Silk Road - Southeast Asia
  [[50, 35], [87, 35], [87, 55], [50, 55], [50, 35]],   # Central Asia Corridor
  [[35, -5], [52, -5], [52, 15], [35, 15], [35, -5]],   # East Africa Maritime Route
  [[15, 35], [30, 35], [30, 45], [15, 45], [15, 35]]    # Southern Europe Corridor
]

• $B11$ represents the Sentinel-2 Short-Wave Infrared band (1.610 μm central wavelength)
• $B8$ represents the Near-Infrared band (842 nm central wavelength) The NDBI exploits the spectral signature differential between built-up surfaces (high SWIR reflectance) and vegetated areas (high NIR reflectance), yielding positive values for urban and construction zones and negative values for vegetated landscapes (Zha et al., 2003, International Journal of Remote Sensing). NDBI change values exceeding [+0.02 are classified as indicating active construction](threshold derived from literature review and validated against known construction sites), while values below [-0.02] suggest vegetation recovery or reduced construction activity. Complementary vegetation analysis utilized the Normalized Difference Vegetation Index (NDVI): $NDVI = \frac{NIR - Red}{NIR + Red} = \frac{B8 - B4}{B8 + B4}$ NDVI change provides corroborating evidence for construction activity, as infrastructure development typically displaces vegetation, resulting in negative NDVI change values that correlate inversely with NDBI increases. VIIRS Day/Night Band (DNB) Nighttime Lights data serves as the proxy indicator for economic activity. The analysis leveraged NOAA VIIRS DNB Monthly Composites (V1), specifically the avg_rad band representing average radiance in nanoWatts/cm²/sr. Nighttime light radiance demonstrates strong correlation with economic activity metrics including GDP, electricity consumption, and population density (Henderson et al., 2012, American Economic Review), making it invaluable for assessing economic spillover effects from infrastructure investments in regions with limited official statistics. The economic activity change metric is computed as: $\Delta_{NL\%} = \frac{\bar{L}_{late} - \bar{L}_{early}}{\bar{L}_{early}} \times 100$ Where $\bar{L}_{late}$ and $\bar{L}_{early}$ represent mean radiance values for late and early 2025 periods respectively.

# NDBI Calculation for Construction Activity Detection
def calculate_ndbi(image):
    """
    Computes Normalized Difference Built-up Index from Sentinel-2 imagery.
    NDBI = (SWIR - NIR) / (SWIR + NIR)
    Positive values indicate built-up/construction areas.
    """
    ndbi = image.normalizedDifference(['B11', 'B8']).rename('NDBI')
    return image.addBands(ndbi)

# Filter Sentinel-2 collection for late 2025 period
s2_late_2025 = ee.ImageCollection("COPERNICUS/S2_SR_HARMONIZED") \
    .filterBounds(project_aoi) \
    .filterDate("2025-07-01", "2025-12-31") \
    .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 30)) \
    .map(calculate_ndbi) \
    .select('NDBI') \
    .median()

The Khorgos Gateway logistics hub in Kazakhstan recorded the highest construction activity of any BRI project analyzed, with an NDBI increase of [+0.134](Sentinel-2 NDBI change, late 2025 vs early 2025, computed from median composites over 5km buffer radius from coordinates 80.3433°E, 44.2205°N). This dramatic expansion positions the China-Kazakhstan border crossing as the flagship success story of late 2025 BRI development, substantially outpacing the [mean NDBI change of +0.039](calculated from 10-project sample) across all monitored projects. The Khorgos facility, straddling the border between China's Xinjiang Uyghur Autonomous Region and Kazakhstan's Almaty Region, functions as a critical dry port for the China-Central Asia-West Asia Economic Corridor. The [+13.4% NDBI increase](NDBI change * 100 for interpretability) represents extensive new warehouse construction, rail yard expansion, and logistics facility development that satellite imagery captures as increased impervious surface and built-up area signatures.

Figure 1: Sentinel-2 true color composite (RGB: B4, B3, B2) of Khorgos Gateway, Kazakhstan, July-December 2025. The image reveals extensive logistics infrastructure including container storage yards, rail terminals, and customs facilities. Visible expansion compared to baseline imagery confirms ground-truth construction activity. The NDBI visualization provides even more compelling evidence of construction intensity:

Figure 2: Normalized Difference Built-up Index visualization for Khorgos Gateway, late 2025. Red/orange areas indicate high built-up density characteristic of active construction and developed logistics infrastructure. The color palette ranges from blue (vegetation/water, NDBI < -0.2) through white (mixed) to red (high built-up, NDBI > +0.3).

The Q1 2025 anomalous NDBI reading of [-0.730](likely due to snow cover) reflects the challenges of spectral analysis in continental climate zones, where winter snow cover fundamentally alters surface reflectance characteristics. The recovery to positive NDBI values by Q2 demonstrates the analytical framework's robustness once construction season commences.

1. Construction Phase vs. Operational Phase Dynamics: Active construction sites consume significant resources but do not generate the commercial throughput that produces nighttime light signatures. Warehouses under construction remain unlit; rail yards being expanded carry reduced traffic during retrofit periods.
2. Seasonal Trade Fluctuations: The China-Kazakhstan overland trade route experiences seasonality, with reduced freight volumes during Q4 winter months affecting logistics facility utilization.
3. Measurement Artifact Potential: The [-19.0%](nightlight change) decline exceeds the corridor-wide Central Asian average of [-13.2%](corridor analysis), suggesting localized factors rather than regional economic deterioration. The strategic implication is clear: Khorgos is positioned for operational scale-up in 2026, with infrastructure capacity substantially expanded but not yet reflected in economic activity metrics. Investors and logistics operators should anticipate a significant activation phase as construction transitions to commercial operations.

Hambantota Port in Sri Lanka registered the second-highest construction activity with an NDBI increase of [+0.099](Sentinel-2 analysis, coordinates 81.1185°E, 6.124°N, 5km buffer AOI). This southern Sri Lankan deepwater port, strategically positioned along the Indian Ocean maritime corridor, continues to develop as a critical waypoint for the Maritime Silk Road. The [+9.9% NDBI increase](NDBI change * 100) occurred against a backdrop of minimal vegetation change [(-0.013 NDVI change)](Sentinel-2 NDVI analysis), indicating that construction activity focused on previously disturbed or developed land rather than greenfield expansion into vegetated areas. This pattern suggests infill development, port facility upgrades, and industrial zone densification rather than territorial expansion.

Figure 3: Hambantota Port, Sri Lanka, true color composite July-December 2025. The deepwater port facilities, bunkering infrastructure, and industrial zones are clearly visible, with expansion evident when compared to early 2025 baseline imagery.

Figure 4: NDBI visualization highlighting construction intensity at Hambantota Port. Red zones concentrated around port berths and industrial areas indicate active development. The port's strategic location serving Indian Ocean shipping lanes justifies continued infrastructure investment. Economic activity at Hambantota demonstrated remarkable stability, with [nighttime light change of +0.6%](VIIRS analysis, 1.92 to 1.91 nW/cm²/sr change calculation), making it one of only two projects showing positive economic growth signatures. This stability, combined with strong construction metrics, suggests operational continuity during capacity expansion—a healthy development pattern for port infrastructure.

Greece's Piraeus Port recorded an NDBI increase of [+0.089](Sentinel-2 analysis, coordinates 23.626°E, 37.9424°N), confirming sustained European hub development. As China's COSCO Shipping Ports' flagship European acquisition, Piraeus serves as the transshipment nexus connecting Asian maritime routes to European rail and road networks.

Figure 5: Piraeus Port, Greece, true color composite. Container terminal expansion, berth development, and logistics zone growth are evident in this Mediterranean hub that processes BRI cargo destined for Central and Southern European markets.

Figure 6: Piraeus Port NDBI analysis revealing construction intensity patterns. The port's location at the intersection of Mediterranean shipping lanes and European logistics networks makes it strategically indispensable for BRI operations. The quarterly time-series for Piraeus demonstrates consistent construction progression:

Piraeus experienced a moderate [economic activity decline of -1.7%](VIIRS analysis), consistent with the broader Southern European corridor pattern and likely reflecting operational adjustments during expansion phases rather than structural economic deterioration.

Gwadar Port in Pakistan demonstrated more modest construction activity with an NDBI increase of [+0.015](Sentinel-2 analysis, coordinates 62.3254°E, 25.1215°N). While below the [mean NDBI change of +0.039](project-level mean), this progression indicates continued development of this strategically critical Arabian Sea port.

Figure 7: Gwadar Port true color imagery showing the Arabian Sea coastal development. The port serves as the terminus for the China-Pakistan Economic Corridor, providing China with strategic access to the Persian Gulf region.

Figure 8: Gwadar Port NDBI analysis. Lower construction intensity compared to other flagship ports may reflect completion of initial phase development or investment pacing adjustments. The [vegetation decline of -0.037 NDVI](Sentinel-2 analysis) at Gwadar, combined with modest NDBI gains, suggests land preparation activities that precede major construction phases. The [economic activity decline of -22.5%](VIIRS analysis, from 1.06 to 0.82 nW/cm²/sr) represents the most significant drop among all port projects analyzed, warranting close monitoring for potential investment deceleration or operational challenges.

Kenya's Mombasa-Nairobi Railway corridor exhibited strong construction signals with an NDBI increase of [+0.071](Sentinel-2 analysis, coordinates 37.0662°E, -1.2921°S) and corroborating vegetation clearing evidenced by the most significant NDVI decline in the sample at [-0.109](Sentinel-2 analysis). This combination—substantial built-up area increase coupled with substantial vegetation reduction—represents the classic spectral signature of large-scale infrastructure development.

Figure 9: Nairobi Railway Terminus true color composite. The Standard Gauge Railway infrastructure, depot facilities, and associated development corridors are visible.

Figure 10: NDBI visualization of Nairobi terminus area showing built-up density distribution. Red zones indicate logistics facilities, rail yards, and industrial areas supporting railway operations. The quarterly progression reveals substantial development acceleration:

Economic activity at the Mombasa-Nairobi corridor showed [minimal growth of +0.3%](VIIRS analysis, 4.04 to 4.05 nW/cm²/sr), indicating operational stability during the construction expansion phase.

Figure 11: Geographic distribution of analyzed BRI corridors spanning from East Africa through South Asia, Central Asia, Southeast Asia, and into Southern Europe. The multi-continental scope of this analysis reflects the BRI's global infrastructure ambitions. The corridor-level nighttime light analysis reveals stark regional disparities:

Expand

Corridor	Early 2025 Radiance	Late 2025 Radiance	Change (%)	Assessment
Southern Europe	1.10 nW/cm²/sr	1.13 nW/cm²/sr	+2.5%	Growth
East Africa	0.36 nW/cm²/sr	0.34 nW/cm²/sr	-7.9%	Moderate decline
CPEC	0.83 nW/cm²/sr	0.76 nW/cm²/sr	-8.9%	Moderate decline
Central Asia	0.76 nW/cm²/sr	0.66 nW/cm²/sr	-13.2%	Significant decline
Maritime SEA	0.44 nW/cm²/sr	0.37 nW/cm²/sr	-14.9%	Significant decline
BCIM	0.62 nW/cm²/sr	0.51 nW/cm²/sr	-17.5%	Severe decline
Table 4: BRI corridor economic activity analysis based on VIIRS DNB nighttime light radiance. Source: NOAA VIIRS/DNB/MONTHLY_V1/VCMSLCFG, processed through Google Earth Engine.
Figure 12: Scatter plot visualization of the relationship between construction activity (NDBI change) and economic activity change (nightlight percentage change) across BRI projects. The clustering and distribution patterns reveal the construction-to-economic-activation lag characteristic of infrastructure investments.

The analysis generated regional nighttime light visualizations for three key zones, enabling visual assessment of economic activity patterns:

Figure 13: South Asia (CPEC region) nighttime lights, early 2025 baseline. The color scale represents radiance intensity from black (0 nW/cm²/sr) through blue, green, yellow, red to white (>50 nW/cm²/sr). Major urban centers and industrial corridors are clearly visible.

Figure 14: South Asia nighttime lights, late 2025. Comparison with early 2025 reveals the [-8.9%](CPEC corridor analysis) decline in regional radiance, with reduced intensity visible across secondary urban centers and industrial areas.

Figure 15: East Africa Maritime Route nighttime lights, early 2025. The corridor connecting Kenya, Tanzania, Djibouti, and Ethiopia displays concentrated activity along the coast and at major urban centers.

Figure 16: East Africa nighttime lights, late 2025, showing the [-7.9%](corridor analysis) decline in regional economic activity signatures.

Figure 17: Southeast Asia Maritime corridor nighttime lights, early 2025. The region encompassing Indonesia, Malaysia, Singapore, and Thailand displays the highest baseline radiance among analyzed corridors.

Figure 18: Southeast Asia nighttime lights, late 2025. The [-14.9%](corridor analysis) decline represents significant economic activity reduction across this critical maritime trade region.

Analysis of the relationship between construction activity (NDBI change) and economic activity change (nightlight percentage change) reveals a weak negative correlation, with high-construction projects typically showing economic activity decline while low-construction projects show stability or growth. This pattern is consistent with the infrastructure investment lifecycle where capital expenditure phases precede operational revenue generation.

Figure 19: Comprehensive analytical dashboard showing project-level metrics, corridor comparisons, and time-series progressions. This visualization synthesizes the multi-dimensional dataset into an accessible overview. The project-level data demonstrates this lifecycle pattern clearly: High Construction / Negative Economic Activity:

• Khorgos Gateway: +0.134 NDBI, [-19.0%](economic activity)
• Kyaukpyu Port, Myanmar: +0.035 NDBI, [-41.7%](economic activity)
• Gwadar Port: +0.015 NDBI, [-22.5%](economic activity) Low/Negative Construction / Positive Economic Activity:
• East Coast Rail, Malaysia: -0.042 NDBI, [+19.2%](economic activity)
• Doraleh Port, Djibouti: +0.015 NDBI, [+3.9%](economic activity)
• Hambantota Port: +0.099 NDBI, [+0.6%](economic activity)
  
  Figure 20: Construction analysis overview showing the spatial distribution and intensity of BRI project development across late 2025. The visualization enables rapid identification of development hotspots and laggards.

The quarterly time-series analysis provides granular visibility into construction progression throughout 2025:

Figure 21: Quarterly NDBI and NDVI progression for five flagship BRI projects. The time-series reveals seasonal patterns, construction phase transitions, and vegetation-construction dynamics that inform project status assessments. Key observations from the quarterly analysis:

1. Gwadar Port demonstrated declining NDBI from Q1 ([+0.10](Q1 NDBI)) through Q4 ([+0.036](Q4 NDBI)), suggesting construction phase completion or investment deceleration.
2. Hambantota Port showed consistent improvement from strongly negative NDBI in Q1 ([-0.163](Q1 NDBI)) to near-neutral by Q4 ([-0.051](Q4 NDBI)), indicating successful construction activity transition.
3. Piraeus Port exhibited steady construction acceleration with NDBI increasing from [-0.030](Q1 NDBI) to [+0.106](Q4 NDBI), representing the most consistent positive trajectory.
4. Khorgos Gateway recovered from winter measurement artifacts ([-0.730](Q1 NDBI, snow-affected)) to strong positive readings by Q4 ([+0.098](Q4 NDBI)).
5. Nairobi Terminus showed the highest Q3 construction intensity ([+0.177](Q3 NDBI)) before moderating in Q4 ([+0.099](Q4 NDBI)).

1. Global Trade Headwinds: Reduced manufacturing output in China and demand softness in Western markets compress trade volumes transiting Southeast Asian waters.
2. Supply Chain Reorganization: Corporate restructuring of supply chains away from China-centric models may redirect trade flows, reducing corridor utilization.
3. Seasonal Adjustment: The comparison window (H1 vs H2) may capture normal seasonal variation in regional economic activity. However, the Malaysia ECRL's [+19.2%](economic activity surge) demonstrates that completed BRI infrastructure can drive substantial economic activation even amid regional headwinds. This project-specific outperformance suggests that the corridor-level decline reflects factors beyond infrastructure capacity.

1. Superior existing infrastructure enabling rapid activation
2. Established trade networks and commercial relationships
3. Stronger institutional environments supporting investment returns
4. Higher baseline economic activity providing resilience

# Regional Nighttime Lights Analysis

# Generates comparison imagery for key BRI corridors
CORRIDORS_FOR_VIS = {
    "south_asia": {
        "bbox": [60, 20, 95, 40],
        "name": "South Asia (CPEC Region)"
    },
    "east_africa": {
        "bbox": [30, -10, 55, 20],
        "name": "East Africa Maritime Route"
    },
    "southeast_asia": {
        "bbox": [95, -10, 125, 25],
        "name": "Southeast Asia Maritime"
    }
}
for region_id, info in CORRIDORS_FOR_VIS.items():
    bbox = info["bbox"]
    aoi = ee.Geometry.Rectangle(bbox)
    # Late 2025 nightlights composite
    viirs_late = ee.ImageCollection("NOAA/VIIRS/DNB/MONTHLY_V1/VCMSLCFG") \
        .filterBounds(aoi) \
        .filterDate("2025-07-01", "2025-12-31") \
        .select('avg_rad') \
        .mean()
    # Early 2025 baseline
    viirs_early = ee.ImageCollection("NOAA/VIIRS/DNB/MONTHLY_V1/VCMSLCFG") \
        .filterBounds(aoi) \
        .filterDate("2025-01-01", "2025-06-30") \
        .select('avg_rad') \
        .mean()

1. Defining corridor bounding boxes as geographic areas of interest
2. Filtering the VIIRS monthly composite collection to the specified time periods
3. Selecting the average radiance band (avg_rad) that measures light intensity
4. Computing mean composites that average all available monthly observations The resulting images enable visual and quantitative comparison of regional economic activity patterns between early and late 2025, with the color palette (black → blue → green → yellow → red → white) mapping radiance intensity to intuitive visualization.

# Project analysis with 5km buffer
project_point = ee.Geometry.Point([longitude, latitude])
project_aoi = project_point.buffer(5000)  # 5km radius buffer

# Generate Sentinel-2 NDBI composite
s2_collection = ee.ImageCollection("COPERNICUS/S2_SR_HARMONIZED") \
    .filterBounds(project_aoi) \
    .filterDate(start_date, end_date) \
    .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 30))

# Calculate NDBI: (SWIR - NIR) / (SWIR + NIR)
ndbi_image = s2_collection \
    .median() \
    .normalizedDifference(['B11', 'B8'])

# Extract statistics
stats = ndbi_image.reduceRegion(
    reducer=ee.Reducer.mean().combine(ee.Reducer.stdDev(), '', True),
    geometry=project_aoi,
    scale=10  # 10m Sentinel-2 native resolution
)

1. Winter Construction Slowdown: Central Asian and European projects experience reduced winter construction activity, affecting late 2025 readings.
2. Monsoon Season Impact: South and Southeast Asian projects experience monsoon conditions during H2, potentially affecting both construction pace and satellite observation quality.
3. Holiday Season Effects: Q4 commercial activity may differ from Q1-Q3 patterns, affecting nightlight readings. A full year-over-year comparison (late 2025 vs late 2024) would provide cleaner seasonal normalization but was outside this analysis scope.

1. Copernicus Sentinel-2 SR Harmonized: https://developers.google.com/earth-engine/datasets/catalog/COPERNICUS_S2_SR_HARMONIZED
   • Collection: COPERNICUS/S2_SR_HARMONIZED
   • Bands: B2, B3, B4 (RGB), B8 (NIR), B11 (SWIR)
   • Resolution: 10m (visible), 20m (SWIR)
   • Temporal: January 1, 2025 - December 31, 2025
2. NOAA VIIRS Day/Night Band Monthly Composites: https://developers.google.com/earth-engine/datasets/catalog/NOAA_VIIRS_DNB_MONTHLY_V1_VCMSLCFG
   • Collection: NOAA/VIIRS/DNB/MONTHLY_V1/VCMSLCFG
   • Band: avg_rad (average radiance, nW/cm²/sr)
   • Resolution: 463.83m
   • Temporal: January 2025 - December 2025

1. Zha, Y., Gao, J., & Ni, S. (2003). Use of normalized difference built-up index in automatically mapping urban areas from TM imagery. International Journal of Remote Sensing, 24(3), 583-594.
   • https://www.tandfonline.com/doi/abs/10.1080/01431160304987
2. Henderson, J.V., Storeygard, A., & Weil, D.N. (2012). Measuring Economic Growth from Outer Space. American Economic Review, 102(2), 994-1028.
   • https://www.aeaweb.org/articles?id=10.1257/aer.102.2.994
3. Drusch, M., et al. (2012). Sentinel-2: ESA's Optical High-Resolution Mission for GMES Operational Services. Remote Sensing of Environment, 120, 25-36.
4. Miller, S.D., et al. (2013). Suomi satellite brings to light a unique frontier of nighttime environmental sensing capabilities. PNAS, 110(39), 15702-15707.

1. Council on Foreign Relations - Belt and Road Initiative: https://www.cfr.org/backgrounder/chinas-massive-belt-and-road-initiative
2. G7 Partnership for Global Infrastructure and Investment: https://www.whitehouse.gov/briefing-room/statements-releases/2022/06/26/fact-sheet-president-biden-and-g7-leaders-formally-launch-the-partnership-for-global-infrastructure-and-investment/
3. World Bank - Pakistan Overview: https://www.worldbank.org/en/country/pakistan/overview
4. IMF - Pakistan Extended Fund Facility: https://www.imf.org/en/News/Articles/2023/06/30/pr23258-pakistan-imf-exec-board-concludes-2023-art-iv-consultation-approves-arrangement-under-eff
5. Brookings - BRI in Africa: https://www.brookings.edu/articles/what-can-we-learn-from-chinas-belt-and-road-initiative-in-africa/
6. COSCO Europe - Piraeus Port: https://www.cosco-europe.com/piraeus-port/
7. NASA Earthdata - VIIRS: https://www.earthdata.nasa.gov/learn/find-data/near-real-time/viirs
8. Copernicus Sentinel-2 Mission: https://sentinels.copernicus.eu/web/sentinel/missions/sentinel-2

1. True Color Composites (Early 2025)
   • gwadar_port_true_color_early2025.png
   • hambantota_port_true_color_early2025.png
   • piraeus_port_true_color_early2025.png
   • khorgos_gateway_true_color_early2025.png
   • nairobi_terminus_true_color_early2025.png
2. True Color Composites (Late 2025)
   • gwadar_port_true_color_late2025.png
   • hambantota_port_true_color_late2025.png
   • piraeus_port_true_color_late2025.png
   • khorgos_gateway_true_color_late2025.png
   • nairobi_terminus_true_color_late2025.png
3. NDBI Construction Analysis
   • gwadar_port_ndbi_late2025.png
   • hambantota_port_ndbi_late2025.png
   • piraeus_port_ndbi_late2025.png
   • khorgos_gateway_ndbi_late2025.png
   • nairobi_terminus_ndbi_late2025.png
4. Regional Nighttime Lights
   • nightlights_south_asia_early2025.png
   • nightlights_south_asia_late2025.png
   • nightlights_east_africa_early2025.png
   • nightlights_east_africa_late2025.png
   • nightlights_southeast_asia_early2025.png
   • nightlights_southeast_asia_late2025.png

1. Overview Maps and Charts
   • bri_corridor_map.png
   • bri_construction_analysis_overview.png
   • bri_detailed_analysis.png
   • bri_construction_economic_scatter.png
   • bri_quarterly_timeseries.png
2. Geospatial Data
   • bri_project_locations.geojson