Survey planning

Why this matters

A drone map can be no better than the images behind it. Fly too high and the detail you need is smaller than a pixel. Overlap too little and the software cannot join the images. Fail to agree the coordinate system at the start and the user may open the file to find the map hundreds of metres out of place. None of these can be fixed in the office; the site has to be flown again. Planning is therefore the most valuable step of any survey.

The whole course follows one hypothetical case: surveying a 240 × 160 m materials yard in an industrial estate to measure the yard area and stockpile volumes, and inspecting the solar roof of the building next door. All numbers are synthetic training data, not real survey results. The Python code for every module can be downloaded from /downloads/uat-361/.

Start from the brief, not the flight height

A survey brief is a one-page agreement made before any calculation. It answers six questions.

Thailand lies in UTM zones 47N and 48N (EPSG:32647 and 32648) on WGS 84. Older data may use the Indian 1975 datum, whose coordinates can differ by hundreds of metres. Never guess the EPSG code; ask the user which system their existing data uses. If the user has not set an accuracy requirement, record it as “to be confirmed” rather than promising one from the camera resolution.

GSD and ground footprint

GSD (ground sampling distance) is the size of ground covered by one pixel, like the grain size of the picture. With a 2 cm GSD, objects smaller than 2 cm cannot be resolved as separate detail. For a camera looking straight down on flat ground:

$\text{GSD}=\frac{H{S}_w}{f{N}_w},W=\frac{H{S}_w}{f}$

where $H$ is the camera height above the ground (m), $S_w$ the sensor width (mm), $f$ the real focal length (mm) and $N_w$ the number of pixels across the width. The ground width $W$ is the footprint. Pix4D stresses that the real focal length must be used, not the 35 mm equivalent.

Overlap and flight lines

Mapping software must see the same point in several images, so images must overlap in two directions. Front overlap ($o_f$) is the overlap between consecutive images along a line; side overlap ($o_s$) is the overlap between images on neighbouring lines.

$B=W_{\text{along}}(1-o_f),D=W_{\text{across}}(1-o_s)$

$B$ is the distance between photo stations and $D$ the distance between flight lines. Planning software such as QGroundControl (Survey pattern) computes these once you choose a camera and overlaps, but the engineer must be able to check that the numbers make sense.

import math

SENSOR_W, SENSOR_H, FOCAL, PX_W = 13.2, 8.8, 8.8, 5472    # mm, mm, mm, pixels
AREA_ALONG, AREA_ACROSS = 240, 160                        # m
FRONT, SIDE, SPEED = 0.80, 0.70, 5.0

for h in (60, 80, 100):
    gsd_cm = h * SENSOR_W / (FOCAL * PX_W) * 100
    across, along = h * SENSOR_W / FOCAL, h * SENSOR_H / FOCAL
    base, spacing = along * (1 - FRONT), across * (1 - SIDE)
    lines = math.ceil(AREA_ACROSS / spacing) + 1
    per_line = math.floor(AREA_ALONG / base) + 1
    path_m = lines * AREA_ALONG + (lines - 1) * spacing
    print(f"H {h:>3} m: GSD {gsd_cm:.2f} cm, footprint {across:.0f} x {along:.0f} m, "
          f"base {base:.0f} m, spacing {spacing:.0f} m, {lines} lines x {per_line} = {lines * per_line} photos, "
          f"{path_m / SPEED / 60:.1f} min, trigger every {base / SPEED:.1f} s")

H  60 m: GSD 1.64 cm, footprint 90 x 60 m, base 12 m, spacing 27 m, 7 lines x 21 = 147 photos, 6.1 min, trigger every 2.4 s
H  80 m: GSD 2.19 cm, footprint 120 x 80 m, base 16 m, spacing 36 m, 6 lines x 16 = 96 photos, 5.4 min, trigger every 3.2 s
H 100 m: GSD 2.74 cm, footprint 150 x 100 m, base 20 m, spacing 45 m, 5 lines x 13 = 65 photos, 4.6 min, trigger every 4.0 s

Terrain and flight patterns

The height in the formula is the height above the ground being imaged, not above the take-off point. If the ground rises 20 m while the drone stays at 80 m, the real height is 60 m and the footprint shrinks to 90 × 60 m, yet the photo spacing is still 16 m and the line spacing still 36 m, so overlap falls.

SENSOR_W, SENSOR_H, FOCAL = 13.2, 8.8, 8.8
BASE, SPACING = 16, 36                  # planned at 80 m above flat ground
for ground_rise in (0, 10, 20, 30):
    h = 80 - ground_rise
    across, along = h * SENSOR_W / FOCAL, h * SENSOR_H / FOCAL
    front, side = 1 - BASE / along, 1 - SPACING / across
    print(f"ground +{ground_rise:>2} m -> height {h} m: front {front:.0%}, side {side:.0%}")

ground + 0 m -> height 80 m: front 80%, side 70%
ground +10 m -> height 70 m: front 77%, side 66%
ground +20 m -> height 60 m: front 73%, side 60%
ground +30 m -> height 50 m: front 68%, side 52%

Choose the flight pattern to suit the site:

• Survey (grid) for polygon areas such as our yard
• Corridor scan for long, narrow areas such as roads, canals or power lines, defined by a centre line and a width
• Oblique images or crossing lines in two directions when building sides must be seen or a better 3D model is needed, at the cost of more time and more images

Before flying, check the CAAT requirements for registration, permission, restricted areas and height (see UAT 313), and obtain the landowner’s permission. The numbers in this module show geometry; they are not authorised flight levels.

Module lab

Common mistakes

Summary

• Start from the brief: user, deliverables, coordinate system, criteria and limits, before computing the plan
• GSD = H·Sw/(f·N) describes the size of detail, not coordinate accuracy
• Photo spacing and line spacing come from the footprint multiplied by (1 − overlap)
• Rising terrain reduces the real height and the overlap, so choose the pattern and height accordingly

Check your understanding

1. A camera with $S_w$ = 13.2 mm, $f$ = 8.8 mm and 5472 pixels flies at 100 m. What is the GSD?
2. The along-track footprint is 80 m with 75% front overlap. What is the photo spacing?
3. The across-track footprint is 120 m with 60% side overlap. What is the line spacing?
4. With a 16 m photo spacing at 8 m/s, how often must the camera fire?
5. Why does hilly terrain need terrain following or extra overlap?