ORTHOMOSAICS, PHOTOGRAMMETRY & NDVI DELIVERABLES
What do you actually get when you hire a drone pilot to do mapping and imaging? It’s all about the deliverables.
Construction, agriculture, surveying, real estate; these are just a few industries that have been revolutionized by using Unmanned Aerial Vehicles (UAVs) or drones. But what exactly do drones provide these industries? Drones are definitely not lifting steel beams or posting properties on Zillow (yet). It’s the unique data you can get from flying drones (and not all types of UAVs or flight plans will provide the same data) that provides benefits in all these fields. In this post I’m going to explain exactly what information you can get from a drone, what type of drone and pilot to use, and provide examples taken from several flights Aerial Hawks did for our local public park in Loveland, OH. We also have examples from a spillway construction site.
Unfortunately there’s some vocabulary you need to learn before we can jump in, but don’t worry. All you need to know is a bit about GPS and creating maps from images.
Global Positioning System (Global Navigation Satellite System)
GPS is pretty simple in concept. There are a bunch of satellites in the sky from different countries and continents (US, Russia, Europe, and China, etc.) and those satellites orbit the Earth at around 12,000 miles (20,000 km). Each nation calls their own satellites a different name: GPS for the US, GLONASS for Russia, Galileo for Europe, and Beidou for China, for example. These different satellite systems are called constellations, and the collective of all constellations is the Global Navigation Satellite System, or GNSS. At any given time and location on Earth there’s probably at least 25 satellites in the sky above you, all from different constellations[1].
Since each satellite is constantly broadcasting information about its local time and positioning, a GPS unit such as the one in your phone, in a drone, or in surveying equipment just needs to obtain information from 4 different satellites to triangulate its position on the Earth’s surface. The details are a bit more complicated and additional technologies such as RTK are often used to obtain more accurate results, but the basic process of using GPS is the same for all receivers: receive radio signals from a bunch of different satellites in the sky above you and use that data to triangulate your position.
Maps, Orthomosaics, and Photogrammetry
Imagine you are the project manager of a construction site, and you need to figure out how much dirt is in a stockpile or how to plan cut/fill operations. You need some sort of map of your site to communicate exactly where to dig, and you need to be able to measure distances accurately. If you just flew a drone above your field, construction site, or city street and took a picture, you’d have a “map” of the area. But everything would be stretched and distorted because you’re looking at the sides of other buildings, the ground would squeeze as you looked toward the horizon, and the map would be effectively useless because you couldn’t measure distance accurately or see things from a top-down perspective. Everything farther away from the camera would look smaller, and everything close to the camera would look too big. The problem with generating maps from pictures is fundamentally perspective.
This drone photo of a construction site looks nice, but the cars closest to the camera are much bigger than the ones farther away. It’s not a picture you could use to make measurements
In order to fix this, we could zoom out the camera as far as possible and point the camera straight down so that the angle between different buildings was as small as possible. But then we’d encounter another problem: the Earth isn’t flat (sorry conspiracy theorists!). If we zoom out our camera far enough to reduce squashing and stretching, we’d have to account for the curvature of the earth somehow.
The answer to these issues is orthorectification. Orthorectification is usually accomplished by taking many overlapping images from a drone flight and processing them with specialized software such as Pix4D to remove the perspective. The software is effectively creating a new image that appears to be fully top-down at every spot, an orthomosaic map.
The new orthomosaic map, after being validated with some calibration techniques used in surveying (Ground Control Points, etc) can be used to measure real distances. The act of measuring based on images is called photogrammetry, and it’s an entire field on its own. We’re just dipping our toes into the ocean of photogrammetry here to demonstrate the usefulness of orthomosaic maps.
Real-Life Example #1: Park
Here’s an orthomosaic Aerial Hawks created at Home of the Brave Park in Loveland, OH, USA.
We’re using DroneDeploy in this instance, and you can see that I’ve drawn a line across the baseball infield to measure its width (182.19ft). This is only possible because the map is a top-down perspective from every angle due to the orthorectification process. Here’s another example where we can measure the area of the restroom building’s roof (583ft2):
You can be confident in the accuracy of these measurements because you’re measuring using a strictly top-down (nadir) perspective thanks to orthorectification instead of the oblique perspective you would get with a simple picture. Due to their accuracy and orthorectified perspective, orthomosaics are the starting point for creating Plant Health maps and can be used to validate and cross-reference results obtained from 3D Meshes and elevation maps.
Real-Life Example #2: Spillway Construction Site
This image of a spillway construction site was taken by Aerial Hawks at an altitude of 100ft. Although this image is useful for progress monitoring, marketing, or quality assurance purposes, it is not suitable for making measurements. Due to the perspective of the drone, the outside edges of the walls are not visible, and the triangular waffling near the lake obscures the inside of the spillway.
Orthorectification is the process of stitching together hundreds of overlapping images via sophisticated software such as Pix4D and DroneDeploy to create an orthomosaic map that removes the effects of perspective. Because each image is taken from a different angle, the software can create a new image where the perspective is straight down (nadir) at every point.
Here is an orthorectified map of the same spillway that was stitched together from 170 different images taken from an altitude of 160ft of the site using a gridded flight plan. This map could be used to make measurements of distances, volumes, and areas because it has a top-down perspective at every point in the image. You’ll notice that we can see the inside of the waffling, both edges of the walls, and other details that were obscured by the perspective of the single photo.
What kind of precision can you get from drones?
Due to the low flight altitude, the above map’s Ground Sampling Distance (a measure of its precision) is 0.6in/pixel, which corresponds to potential errors of 1.2in. to 2.4in., within survey-grade. Greater precision is possible (up to 0.4in/pixel, which corresponds to 0.8in/2cm errors) but this takes significantly longer because the drone must fly at a lower altitude and take many more images. Many projects do not require that level of detail, although some do. Aerial Hawks can work with you to determine the right parameters for your project. Most professional land surveyors using traditional methods such as RTK can guarantee measurements around 1cm (0.38in) horizontally and 2cm (0.78in) vertically.
Orthomosaic maps are often used in legal contexts due to their high relative and global coordinate positioning. Aerial Hawks is not a licensed land surveyor, but we work with licensed professionals to verify the accuracy of our results when they are being used in legal contexts. Our drone technology dramatically speeds up the surveying timeline from weeks to mere days, and provides information that a surveyor could not generate on their own, such as high-quality 3D models and photography/videography. We do not replace surveyors, we just allow them to focus on verifying end results for regulatory purposes instead of spending weeks in the field. We make their work quicker, easier, and more efficient, which benefits your project’s budget and schedule.
Back to deliverables:
So when you hire a drone pilot to fly, they’ll send you all the pictures they took of your property, construction site, etc. And high-resolution aerial shots are fantastic, but if you’re looking to do site planning, you need something a bit more precise. So another deliverable drone pilots will give you is a fully processed orthomosaic map of your property, construction site, etc. Aerial Hawks make these files available in the cloud for you to download, and send you several different copies of the same map in different file formats compatible with various tools you might use (ArcGIS vs AutoCAD for instance).
Also, orthomosaic maps aren’t the only processed deliverable we can create. For instance, quality assurance crews and superintendents might want a fully 3D model of a site so they can check that safety standards are being followed and the project as-built matches the blueprints. You need a 3D textured mesh, which can also be created using photogrammetry techniques.
A third class of deliverables are orthomosaic maps with plant health or elevation data. Plant health can be determined using normalized difference vegetation index (NDVI) algorithms to analyze infrared light using a special camera. Topographic maps are useful to project managers who need information about land grading or water flow.
The big picture...
Hiring a drone pilot involves more than just taking high-quality pictures and video, although Aerial Hawks’ pilots are experts at photography too! Processing the data, generating useful maps and deliverables allow you to gain new insights into your project, whatever it may be.
References:
Rizos, Chris & Higgins, Matt & Johnston, Gary. (2010). Impact of Next Generation GNSS on Australasian Geodetic Infrastructure