DDA UAV & Topographic Surveys — 1,000 acres

Project Overview

For a further set of DDA assignments in Delhi covering about 1,000 acres, BTPL combined UAV photogrammetry with conventional ground survey. Aerial capture gave continuous coverage of each area together with an orthomosaic record of the land as it stood, while total station and RTK observation supplied what the camera could not see and confirmed detail on the ground. Both datasets were held on one GNSS control framework and issued as a single terrain model, contour and CAD set for each area.

What did DDA require?

Over areas of this size, a continuous surface and a photographic record of the land carry information that a coded line drawing does not. The orthomosaic shows occupation and land use across the whole parcel as a picture a planner can read directly, while the terrain model supports level, drainage and earthwork assessment over the full extent and not only at the points a surveyor was able to occupy. Aerial capture also covers ground that cannot be walked freely, so the record extends to the parcel boundary. What it sees there is the top of whatever is standing: under closed canopy the terrain beneath still has to be established from the ground, and boundaries, drain inverts and structures are not reliably measurable from imagery alone. The two methods were therefore run together.

What made this project challenging?

• UAV operation in an urban environment, where airspace clearance, restricted zones, surrounding development and third-party safety on the ground govern where and when flying is possible at all.
• Ground that is built over or occupied, where the photogrammetric surface represents rooftops, structures and stored material and has to be separated out before a terrain model is trustworthy.
• Detail that photogrammetry cannot resolve — drain inverts, chamber and service evidence, boundary walls in shadow, and ground under canopy — requiring supplementary ground measurement on the same control framework.

How did BTPL carry out the survey?

Before any flying, BTPL established a GNSS control framework over each area, observing pre-marked ground control and independent check points by DGPS and RTK and marking them so they remained identifiable in the aerial imagery. Each area was then flown as planned photogrammetric blocks with forward and side overlap, with block layout and flight lines set against the clearances in place and the obstructions present. Ground survey followed on the same control: total station and RTK observation of boundary lines and walls, structures, drains and inverts, visible service evidence and the ground beneath canopy, together with independent spot levels reserved for testing the classified surface. Aerial and ground data were then merged so that each parcel is represented by one terrain model and one drawing set.

How was the data processed?

Each block's point cloud was built from imagery triangulated onto the surveyed ground control and then densely matched. From that point the work is largely editorial: the cloud has to be separated into bare ground and everything standing on it — vegetation, buildings, boundary walls, stored material — and the automatic classification was corrected by hand over canopy, built-over and disturbed ground, where it is least dependable. The terrain model and orthomosaic were derived from the edited cloud, and contours from the model. Planimetric detail digitised off the orthomosaic was set against the ground-surveyed strings and reconciled with them before the CAD drawing was closed out.

How was accuracy verified?

RTK spot levels observed under canopy and across built-over and disturbed ground were set against the classified terrain at those points, one by one. Ground points held back out of the aerial adjustment gave the general check on each block. Overlap between adjoining blocks was examined for continuity of level and feature so that a merged area carries no step at the joins. Detail digitised from the imagery was compared with the ground-surveyed strings, and boundary and structure lines read off the orthomosaic were confirmed by field observation before being accepted into the drawing.

What was delivered?

Four products on one set of coordinates cover each area: orthomosaic, bare-earth terrain model, contours and a topographic CAD drawing, over roughly 1,000 acres in total across the assignments. Holding a picture of the land and a measured surface on the same coordinates allows layout, levels, drainage and earthwork to be studied on the same base that shows existing occupation and land use. The orthomosaic additionally stands as a visual record of each parcel as it was found at the time of survey.