Transmission-line corridor routing and encroachment detection

Satellite and spaceborne LiDAR data identify least-cost transmission routes before ground teams mobilise, then watch for encroachment continuously. Cloud cover and remote terrain are not excuses when SAR coherence and GEDI canopy heights are in the stack.

Sensors

NASA GEDI (ISS-mounted full-waveform LiDAR): Provides canopy height and vertical structure at approximately 25-metre footprint spacing along orbital tracks between 51.6° N and S. Absolute height accuracy is around 1 metre RMSE on gentle terrain; degrades on slopes above 20°. Revisit is non-deterministic and track-dependent, typically weeks to months for a given point, so GEDI is best used for corridor planning and baseline canopy models rather than weekly monitoring.
Sentinel-1 SAR (C-band, ESA): Provides 10-metre resolution Interferometric Wide Swath imagery with 6-day revisit at mid-latitudes (12 days globally, improved by combining ascending and descending passes). Coherence change detection identifies new structures, cleared areas or regrowth within a corridor regardless of cloud cover or darkness. The C-band wavelength (5.6 cm) interacts with vegetation canopy and surface roughness, making it sensitive to changes at the scale of a new access track or a construction compound.
Planet Dove (PlanetScope): 3-metre resolution optical imagery with near-daily revisit globally. Useful for visual confirmation of encroachment alerts flagged by SAR, and for mapping access roads, new structures and land-use change in cloud-free conditions. Four-band (RGBNIR) and SuperDove eight-band variants support basic vegetation health assessment within the right-of-way.
Airbus Pléiades Neo: 30-centimetre native resolution optical tasking. At this scale individual structures, vehicle types and construction equipment are identifiable. Used for high-confidence encroachment adjudication where SAR or medium-resolution optical has flagged an anomaly and a regulatory or legal record is needed. Stereo pairs yield digital surface models accurate to roughly 50 cm in height, supplementing GEDI where airborne LiDAR is unavailable.

What the corridor planner actually needs to know before breaking ground

Routing a high-voltage transmission line is a multi-constraint optimisation problem. Planners must minimise total length while avoiding protected areas, steep slopes, flood zones, existing infrastructure conflicts and populated land parcels. Historically this required expensive airborne surveys and months of desktop GIS work. Spaceborne data compresses that timeline significantly, though it does not eliminate field verification.

GEDI canopy height models give planners a first-pass view of vegetation structure across a candidate corridor at continental scale. Combined with a digital terrain model (the Copernicus DEM at 30-metre posting is publicly available and adequate for initial screening), analysts can estimate where line sag clearance requirements will be hardest to achieve and where tree-felling costs will dominate the civil budget. The honest caveat: GEDI's non-uniform sampling means some sections of a proposed corridor may have sparse or no coverage, requiring interpolation or supplementary airborne LiDAR for the final engineering model.

Clearance margins are not one number. They depend on voltage class and jurisdiction.

This is the detail that generic remote-sensing marketing skips. Minimum ground clearance requirements under energised conductors vary by voltage class and national standard. Under IEC 60071 and many national grid codes, a 400 kV line requires a minimum clearance of around 8 to 10 metres above ground or vegetation, while a 132 kV line may require 5 to 7 metres, depending on the jurisdiction and the conductor's maximum operating temperature sag. In practice, utilities apply a safety buffer on top of the regulatory minimum.

This means the encroachment threshold is not a fixed height. An analyst monitoring a corridor must know the voltage class, the applicable national standard and the conductor sag model for the span in question before declaring a tree a hazard. Satellite data defines the physical geometry. The regulatory threshold is a separate input that must come from the client's engineering team and be encoded in the analysis parameters before any alert is generated.

SAR coherence: the method that works when clouds do not clear

Sentinel-1 coherence change detection compares the phase relationship between two SAR acquisitions of the same area. Where the surface has not changed, coherence is high. Where vegetation has grown, been felled, or a structure has been erected, coherence drops. At 10-metre resolution with a 6 to 12-day repeat, this creates a near-continuous change record that is independent of cloud cover, which matters enormously in tropical corridors where optical revisit is effectively seasonal.

The method has well-documented limits. Dense forest canopy decorrelates quickly even without change, making it harder to distinguish genuine encroachment from normal phenological variation. Dry-season versus wet-season comparisons require careful baseline selection. New metallic structures (pylons, fences, storage tanks) show high coherence and bright backscatter, making them easier to detect than organic encroachment. A well-designed monitoring workflow pairs coherence change with backscatter intensity change to separate these cases.

For corridor monitoring specifically, the practical minimum detectable change is roughly one to two Sentinel-1 pixels, meaning structures or clearings of around 10 to 20 metres in their smallest dimension. Smaller encroachments, such as a single tree leaning into the sag zone, require either airborne LiDAR or a high-resolution optical task to confirm.

Integrating GEDI with SAR for an ongoing clearance model

GEDI provides the canopy height baseline. SAR coherence detects change from that baseline. The combination allows an analyst to ask: has vegetation in this span grown since the baseline survey, and if so, by how much relative to the clearance threshold? In practice this requires co-registering the GEDI footprint data to the SAR grid and interpolating canopy height across the corridor, which introduces uncertainty that should be reported honestly to the client rather than hidden in a confidence score.

Where a corridor passes through terrain with slopes above roughly 20 degrees, GEDI height estimates degrade because the full-waveform return conflates slope and canopy height. In these sections, Pléiades Neo stereo DSMs or dedicated airborne LiDAR remain the more reliable source. Satellize flags these terrain-sensitive zones explicitly in corridor analysis outputs rather than applying a uniform method across heterogeneous terrain.

Third-party construction: the encroachment type utilities most often miss

Vegetation encroachment is slow and follows seasons. Third-party construction, whether an illegal building, a new access road or an agricultural structure, can appear within days. SAR coherence at 6-day revisit catches this class of encroachment reliably, but only if someone is watching. Many utilities run annual or biennial aerial patrols, which means a structure can be partially built before it is detected.

The operational case for continuous satellite monitoring is strongest here. A 6-day coherence pair over a 200-kilometre corridor costs a fraction of a helicopter patrol and produces a spatially complete record. When an anomaly is flagged, a Planet Dove image the following day provides visual context, and a Pléiades Neo task within 48 hours produces a sub-metre record suitable for a legal or regulatory file. The satellite record also establishes the date of first appearance, which matters in enforcement proceedings.

What a monitoring programme looks like in practice

A typical corridor monitoring workflow begins with a one-time baseline: GEDI canopy heights, a Copernicus DEM, and a high-resolution optical mosaic from Planet or Pléiades to map existing structures and land use within the right-of-way buffer. This baseline is ingested into a GIS layer referenced to the corridor centreline and span-by-span clearance thresholds.

After that, Sentinel-1 coherence pairs run on every available acquisition. Change alerts above a configurable threshold trigger an automated optical task request. Confirmed encroachments are logged with coordinates, date of first detection, estimated dimensions and proximity to the nearest span. The output is a GIS alert feed and a monthly encroachment report, not a dashboard that requires a data scientist to interpret. Satellize applies this workflow to open-constellation data and adds commercial tasking where the client holds the appropriate licence. The same analytical architecture that underpins the Tonga crop-estimation programme, where timely change detection drives operational decisions, applies directly to corridor monitoring at any scale.

Typical figures

SAR spatial resolution (Sentinel-1 IW mode)	10 m range × 10 m azimuth (multi-looked GRD product)
SAR revisit (Sentinel-1, mid-latitudes)	6 days (combined ascending + descending); 12 days single geometry
GEDI LiDAR footprint diameter	~25 m; along-track spacing ~60 m; across-track coverage non-uniform
GEDI canopy height accuracy (flat terrain)	~1 m RMSE; degrades on slopes >20°
Pléiades Neo optical resolution	30 cm native; stereo DSM vertical accuracy ~50 cm
Planet Dove optical resolution	3 m (PlanetScope); near-daily global revisit
Minimum detectable surface change (SAR coherence)	~10–20 m in smallest dimension for reliable detection at Sentinel-1 resolution
SAR archive depth (Sentinel-1)	From October 2014 (Sentinel-1A launch); Copernicus Data Space holds full archive
GEDI mission archive	April 2019 to present (subject to ISS operational continuity)
Delivery formats	GeoTIFF canopy height models, GeoPackage or Shapefile alert layers, PDF monthly encroachment report, optional WMS/WMTS feed

Analytics Satellize can run

Least-cost corridor routing model	Multi-criteria GIS cost-surface analysis combining slope (Copernicus DEM), GEDI canopy height, land-cover classification and exclusion zones	Ranked corridor alternatives as GIS vector layers with per-kilometre cost indices
Baseline canopy height map for right-of-way	GEDI L2A/L2B canopy height products co-registered to corridor centreline; gaps filled by interpolation or Pléiades Neo stereo DSM	GeoTIFF canopy height model with uncertainty layer; span-by-span clearance margin table
Coherence change alert layer	Sentinel-1 interferometric coherence differencing between sequential 6- or 12-day pairs; threshold exceedance flagged per corridor segment	Automated GIS alert feed; email or API notification within 24 hours of SAR acquisition processing
Optical confirmation imagery	Planet Dove daily mosaic for rapid visual check; Pléiades Neo tasking for sub-metre adjudication of confirmed alerts	Georeferenced image chips centred on each alert, with metadata (date, resolution, sun angle)
Third-party structure detection log	SAR backscatter intensity change combined with coherence; bright stable returns classified as likely metallic structures	Shapefile of detected structures with date of first appearance, estimated footprint dimensions and distance to corridor centreline
Monthly encroachment report	Aggregation of alert feed, optical confirmation status and historical trend across all monitored spans	PDF report with map appendix, span-by-span status table and prioritised remediation list
Vegetation regrowth rate model	Time-series analysis of SAR backscatter and Planet NDVI within right-of-way to estimate regrowth trajectory and predict next clearance intervention date	Per-span regrowth rate table and forward projection chart; input to maintenance scheduling

Sources

Who does the work

We can get this done for you. Satellize runs its own analyst desk and a strong science team. You do not buy a data feed and work out what it means; our people source the imagery, run the analysis described on this page, and hand you the answer with its confidence limits stated. Discuss this requirement.