Drones Terminology for Survey and Inspection
1. Ground sampling distance (GSD)
The Ground Sampling Distance (GSD) is the distance between two consecutive pixel centers measured on the ground. The bigger the value of the image GSD, the lower the spatial resolution of the image and the less visible details. The GSD is related to the flight height: the higher the altitude of the flight, the bigger the GSD value.
Example:
- A GSD of 10 cm means that one pixel in the image represents linearly 5 cm on the ground (10*10 = 100 square centimeters).
- A GSD of 1 cm means that one pixel in the image represents linearly 10 m on the ground (1*1 = 1 square centimeters).
2. Digital Global Positioning System
3. Ground Control Points
Ground Control Points (GCPs) are defined as points on the surface of the earth of known location used to geo-reference Landsat Level-1 imagery. The Landsat Ground Control Point Search allows you to extract ground control point binary files over your area of interest.
4. GPS Correction Technology
Before jumping into RTK/PPK technologies and its features, it’s necessary to cover GPS correction technology.
GPS correction technology enhances the quality of location data by using global position system receivers to produce precise, accurate data. It’s a technology that’s been used with different tools on the ground and on job sites for years. Only recently has it been offered as an add-on to a survey drone for survey-grade photogrammetry.
Both GCPs and GPS Correction Technology possess the ability to produce the same data; however, GPS Correction Technology doesn’t require the same, timely prep work.
Today, there are currently two technologies driving the conversation around GPS correction technology — Real Time Kinematic (RTK) and Post Processing Kinematic (PPK).
In that same vein, there’s debate on which of the two is a better choice and which you should be looking for to be included on survey drone when shopping for a solution.
5. Real Time Kinematic(RTK) / Post Processed kinematic (PPK)
Real Time Kinematic (RTK) is a GPS correction technology technique that provides real-time corrections to location data when the survey drone is capturing photos of a site.
Post Processed Kinematic (PPK), another GPS correction technology technique that works to correct location data, except in the cloud after the drone data has been captured and uploaded.
Basically, the main difference between the two is when this positional correcting takes place. RTK corrects during the flight, and PPK corrects after flight.
With what we know today, using GPS correctional technology can improve data gathering by 75%. Additionally, it creates smoother workflows because of that time that is shed.
However, it’s important to understand situationally how this all works. Thus prompts the big question — Which one should you choose?
While both technologies are effective, we believe having a survey drone with a PPK attachment is the best route as when deciding between drone solutions that may have one or the other.
Firstly, PPK drones offers more flexibility in terms of the actual flight of the survey drone meaning you have more freedom of how and where the drone is deployed.
In contrast, RTK drones requires a very specific base station and other pieces of equipment that work together in order to process data in real-time.
Secondly, a with PPK drone can refer to previous and future data relative to the current flight, which creates greater dependability. Having this ability ensures the drone is always on track with the flight.
RTK drone has a higher chance of malfunctioning because it cannot retain old or new data to keep the current flight in check. The accountability simply isn’t there, which puts accuracy into question.
Think of it like this:
You’re on a road trip using GPS on your mobile device. You might lose signal a couple times but gain it back quickly.
PPK operates the same on the job site — if there is an issue with its ‘signal’ backup data is there to keep the flight on track.
RTK does not have this ability; if your mobile device worked with an RTK-equivalent GPS, you’d be lost!
GCPs v. PPK/RTK: Which is Best to Receive Fast and Accurate Data?
In drone mapping, buzzwords like PPK, RTK, and GCPs are continually thrown around.
Despite your familiarity with these acronyms, all three share a common ground in drone mapping. However, choosing one over the other can significantly impact the data and how it’s acquired for each survey.
Being extremely accurate and staying on time is important when it comes to ensuring large job site projects are operating both efficiently and profitably. By knowing GCPs and RTK/PPK and their impact, they can have huge implications for both your workflows and profits.
This post is meant to help you explore the different options out there to achieve just that and more with drone mapping.
By the time you’re done reading this, the decision-making process will become much clearer when choosing between GCPs, PPK, and RTK. You’ll understand their differences, how they relate to each other, and which one comes out on top.
Take your guess now to who reigns as best!
Explaining Ground Control Points (GCPs)
Ground control points (GCPs) are points on the surface of the earth of a known location. GCPs are tied in during data processing to georeferenced images from a project and convert ground coordinates of the points to real world locations.
To put it plainly, GCPs allow the survey drone to obtain accuracy down to a sub-centimeter level, which is very precise.
There is one caveat when using GCPs– laying them requires a decent amount of prep work (it can take up to an entire day and a team of people). It’s no clean task, especially when working on a huge site with uneven land.
Given how tech advances nowadays, newer ‘modern’ systematic practices, like drone mapping, can become even more simple and less complicated. In this case, a much more convenient alternative to GCPs called GPS Correction Technology.
Understanding GPS Correction Technology
Before jumping into RTK/PPK technologies and its features, it’s necessary to cover GPS correction technology.
GPS correction technology enhances the quality of location data by using global position system receivers to produce precise, accurate data. It’s a technology that’s been used with different tools on the ground and on job sites for years. Only recently has it been offered as an add-on to a survey drone for survey-grade photogrammetry.
Both GCPs and GPS Correction Technology possess the ability to produce the same data; however, GPS Correction Technology doesn’t require the same, timely prep work.
Today, there are currently two technologies driving the conversation around GPS correction technology — Real Time Kinematic (RTK) and Post Processing Kinematic (PPK).
In that same vein, there’s debate on which of the two is a better choice and which you should be looking for to be included on survey drone when shopping for a solution.
Comparing Real-Time Kinematic and Post Processed Kinematic
Real Time Kinematic (RTK) is a GPS correction technology technique that provides real-time corrections to location data when the survey drone is capturing photos of a site.
Post Processed Kinematic (PPK), another GPS correction technology technique that works to correct location data, except in the cloud after the drone data has been captured and uploaded.
Basically, the main difference between the two is when this positional correcting takes place. RTK corrects during the flight, and PPK corrects after flight.
With what we know today, using GPS correctional technology can improve data gathering by 75%. Additionally, it creates smoother workflows because of that time that is shed.
However, it’s important to understand situationally how this all works. Thus prompts the big question — Which one should you choose?
While both technologies are effective, we believe having a survey drone with a PPK attachment is the best route as when deciding between drone solutions that may have one or the other.
Why to use PPK over RTK
Both these technologies are very similar, however, PPK has a decisive advantage — because of its robustness and consistency.
Let’s analyze this more:
Firstly, PPK drones offers more flexibility in terms of the actual flight of the survey drone meaning you have more freedom of how and where the drone is deployed.
In contrast, RTK drones requires a very specific base station and other pieces of equipment that work together in order to process data in real-time.
Secondly, a with PPK drone can refer to previous and future data relative to the current flight, which creates greater dependability. Having this ability ensures the drone is always on track with the flight.
RTK drone has a higher chance of malfunctioning because it cannot retain old or new data to keep the current flight in check. The accountability simply isn’t there, which puts accuracy into question.
Think of it like this:
You’re on a road trip using GPS on your mobile device. You might lose signal a couple times but gain it back quickly.
PPK operates the same on the job site — if there is an issue with its ‘signal’ backup data is there to keep the flight on track.
RTK does not have this ability; if your mobile device worked with an RTK-equivalent GPS, you’d be lost!
How to transition from GCPs to PPK
If you’re looking to transition from using GCPs to a solution like PPK on your job site, it’s a good idea. The flight planning is significantly reduced since you’re not spending near the amount of time prepping the job site with ground control points.
Make sure you’re working with a solution provider that offers PPK as an option so you can make this smooth transition. They’ll upgrade you with the type of survey drone conducting the flyover that integrates the product. The PPK drone technology will cost a little more when using a subscription, however, with a drone you’ll already have the budget saved.
Along with enhancements in time and product, there will also be a change in spirits of the team on the job site. Not having to spend a whole day laying several GCPs is definitely uplifting and creates a more productive team.
With all of this considered, it’s important to note GCPs still haven’t been completely excluded from drone mapping.
The legacy of GCPs moving forward
When working with technologies like PPK, depending on the circumstance, the concept of ‘ground control’ or ‘ground validation’ isn’t completely irrelevant.
Depending on the size of the site, some ground validation may need to be in place. This allows the survey drone to continue producing a very high level.
You’re still saving an incredible amount of time when working with PPK and ground validation as opposed to working with GCPs. You may need to lay just 1 ground validation point on the entire job site as opposed to 5 to 8 GCPs.
Consider it a housekeeping task that ensures super precise data.
Another situation in which GCPs carry on applicability is when defending data. Since drone mapping is still a relatively new concept, many contractors want to see its precision and accuracy.
GCPs allow the survey drone to produce a very defensible quality report and really prove the validity of the solution.
6. Aerial Imagery
It refers to pictures and videos taken from an aerial perspective. Almost always these will be captured from an aircraft.
7. Ortho Photo / Ortho Rectification / Ortho Mosaics
sing elevation to enable accurate image georeferencing
Imagery has an amazing amount of information, but raw aerial or satellite imagery cannot be used in a GIS until it has been processed such that all pixels are in an accurate (x,y) position on the ground. Photogrammetry is a discipline, developed over many decades, for processing imagery to generate accurately georeferenced images, referred to as orthorectified images (or sometimes simply orthoimages). Orthorectified images have been processed to apply corrections for optical distortions from the sensor system, and apparent changes in the position of ground objects caused by the perspective of the sensor view angle and ground terrain.
An orthomosaic is like Google Earth, but way sharper. It is a large, map-quality image with high detail and resolution made by combining many smaller images called orthophotos.
8. Cloud Points
A point cloud is a set of data points in space. Point clouds are generally produced by 3D scanners, which measure many points on the external surfaces of objects around them. As the output of 3D scanning processes, point clouds are used for many purposes, including to create 3D CAD models for manufactured parts, for metrology and quality inspection, and for a multitude of visualization, animation, rendering and mass customization applications.
9. Digital Surface model
An a LiDAR system, pulses of light travel to the ground. When the pulse of light bounces off its target and returns to the sensor, it gives the range (a variable distance) to the Earth. Hence, how this system earned its name of Light Detection and Ranging.
In the end, LiDAR delivers a massive point cloud filled of varying elevation values. But height can come from the top of buildings, tree canopy, powerlines and other features. A DSM captures the natural and built features on the Earth’s surface.
A DSM is useful in 3D modeling for telecommunications, urban planning and aviation. Because objects extrude from the Earth, this is particularly useful in these examples:
RUNWAY APPROACH ZONE ENCROACHMENT: In aviation, DSMs can determine runway obstructions in the approach zone.
VEGETATION MANAGEMENT: Along a transmission line, DSMs can see where and how much vegetation is encroaching.
VIEW OBSTRUCTION: Urban planners use DSM to check how a proposed building would affect the viewshed of residents and businesses.
10. Digital Terrain Model
A digital terrain model (DTM) actually has two definitions depending on where you live.
In some countries, a DTM is actually synonymous with a DEM. This means that a DTM is simply an elevation surface representing the bare earth referenced to a common vertical datum.
- In the United States and other countries, a DTM has a slight different meaning. A DTM is a vector data set composed of regularly spaced points and natural features such as ridges and breaklines. A DTM augments a DEM by including linear features of the bare-earth terrain.
- DTMs are typically created through stereo photogrammetry like in the example above. For example, contour lines are in purple. The DTM points are regularly-spaced and characterize the shape of the bare-earth terrain.
- In the image above, you can see how the DTM is not continuous and that it’s not a surface model. From these regularly-space and contour lines, you can interpolate a DTM into a DEM. A DTM represents distinctive terrain features much better because of its 3D breaklines and regularly spaced 3D mass points.
11. Digital Elevation Model
A digital elevation model is a bare-earth raster grid referenced to a vertical datum. When you filter out non-ground points such as bridges and roads, you get a smooth digital elevation model. The built (power lines, buildings and towers) and natural (trees and other types of vegetation) aren’t included in a DEM.
When you void vegetation and man made features from elevation data, you generate a DEM. A bare-earth elevation model is particularly useful in hydrology, soils and land use planning
HYDROLOGIC MODELLING: Hydrologists use DEMs to delineate watersheds, calculate flow accumulation and flow direction.
TERRAIN STABILITY: Areas prone to avalanches are high slope areas with sparse vegetation. This is useful when planning a highway or residential subdivision.
SOIL MAPPING: DEMs assist in mapping soils which is a function of elevation (as well as geology, time and climate)
