Aerial Mapping – Millimeter Accuracy for Digitization, Project Development, Surveying and Volume Measurement

Videographics Ingenieurgesellschaft mbH in Sauerlach is specialized in highly precise photogrammetric aerial surveying. We combine the currently most precise GNSS positioning technology with calibrated optical systems (cameras) and a workflow to achieve results with maximum precision and efficiency

Our use cases are digitization of buildings and areas, generating base data for developments, or compare existing data with reality.  In many cases, based on accuracy and cost,  our method is superior over terrestrial surveying methods as well as over most other drone surveying systems. For example, to map an existing road you need to survey a large number of individual ground points to afterwards import them into GIS or CAD applications while you can directly digitize within the aerial generated ortho view. Through simple overlaying ortho view and existing survey plans deviations can be visualized and corrected in an most effective way!

Deviations between drawing and reality can be reviewed most easily

 

Comparison between drawing ad reality. This Example shows a serious deviation. The corner point in the drawing devites by over one meter from it’s real position!

A most effective way for volume determination like in open cast mines, deposals or any other stockpiles. As we do not need any ground control points (GCPs) for georeferencing, in general  there is no need for hiking the territory in order to distribute and painfully survey GCPs! Based on camera calibration data and the exact photo locations (the camera’s focal point) the photogrammetric data processing computes point clouds, 3D models DEMs etc. With almost no additional effort, cross sections or section models can be generated . To calculate volume s above or under a defined plane is just a mouse click away.

3D-model of a small open cast mine with an absolute accuracy of better 20 millimeters

 

digitized grid 5×5 m in .dxf format

 

Since we do not  need and GCPs, highly precise photogrammetric UAV surveys can be executed over remote areas or even in high mountain areas. For these jobs we have special systems who are capable to operate even in altitudes above 2500 m AMSL.

How does Highest Accuracy and Maximum Efficiency Work with Aerial Photogrammetry?

When I decided abou two years ago to invest into a KLAUPPK system, I had quite high expectations regarding the accuracy which should be possible with – according to the manufacturer – the world wide best UAV-GNSS based surveying system. Now, almost 24  months after the first field test with lots of snow on the ground and freezing temperatures we have built up a lot of expertise and an adequate workflow at Videographics Ingenieurgesellschaft which is a paradigm shift in aerial photogrammetry:

We simply have turned the proven but awesome process of GCP-referencing literally upside down. In stead of a few GCPs – and “few” could mean “tens of”  – our system tracks the exact center of exposure  image coordinate of every photo taken – producing many 3D positions. “Many” means a few hundred even with smaller projects! Those 3 D positions are very likely to be more accurate  than any point measured by an terrestrial RTK-System. This is only one, however a very essential part of a system to generate maximum accurate results, since for a well functioning positioning system the following is most important:

  • Satellite Positioning – GNSS (Global Navigation Satellite System)

Our system, KLAUPPK from Australian mapping specialist KLAU Geomatics Ltd. is based on satellite receiver system from Canadian based market leader Novatel. Novatel is part (like e.g. Leica Geosystems) of Hexagon, one of the leading players in remote sensing and geodesy. After the system is powered on and indicates that it sees a sufficient number of satellites it just needs about 5 minutes to get initialized, which means that it can track good quality satellite signals and satellite time (UTC) The most precise timing (millisecond) is essential for data synchronisation in the PPK post processing operation.

 

  • Imaging – precise time stamping of the shutter

Precise registration of the center of exposure time of every photo taken is of highest importance, since the drone records images during its flight and subsequently moves while a photo is taken. An example: If the exposure time is set to 1/1000 second (1ms) and a flight speed of 10 m/sec. (36km) the drone moves ten millimeters. The higher the required resolution (GSD)  the more accurate the timing has to be. This, however, only works with a hardwired signal connection form the mechanical shutter of the camera to the PPK system. Most important is the timestamp of the real shutter event, not e.g. the command via CPU to the camera.

  • Correction- / Reference Data Quality

Most accurate correction data, also known as base station data or reference data are an essential requirement to calculate most accurate positions. If only the satellite observations would be tracked while flying the accuracy of the position quite poor and continuously changing.  In the X and Y  direction (East and North) this even can be several meters. Altitude wise the error could be even more than 10 meters. As a result, this shows also that GNSS coordinates which can be found in the metadata of  images taken from a smartphone or just a “standard2 UAV are highly inaccurate. Furthermore the altitude data of a drone normally is based on barometric changes over its start point.

GNSS Correction Data – Values show need for correction in meters, regarding the antenna base

The correction data represent the deviation between the known exact coordinates of a local fixed GNSS station and the current coordinates calculated form the satellite  observations in all three dimensions.  If these differential values are time synchronized with the satellite observations during the flight, accurate positions can be computed for every observation (dataset). The quality of the correction data is equally important as the accuracy of the GNSS data from the flight. PPK processing (PPK= Post Processed Kinematic) is way more accurate compared to the RTK process (Real Time Kinematic): Die PPK-correction data e.g. provided by SAPOS® are three times more accurate compared to RTK real time data – since this technology relies on a undisrupted LTE connection and requires a certain observation time per survey point.

  • Lever Arm Correction / Compensation of the UAVs Attitude Changes During the Flight

Until now, only the exact position of the antenna base has been calculated, independent from it’s position on the UAV. Through dimensional measurements and integration of the X, Y and Z distances between the camera and the antenna in the processing software the final position of the camera’s focal point can be determined –  as long as the uav is precisely heading North and absolutely leveled. But this is not possible while flying (a multirotor drone controls its speed by different motor speeds which ends up with tilting of the aircraft)  and therefore the continuous attitude changes have to implemented in the position computation (lever arm correction)

Centimeter level accuracy depends on  appropriate lever arm correction

In order to do this the KLAUPPK software utilizes the IMU information of the drone which will get synchronized again over the UTC! This is why we get the accuracy of 3 cm in the focal point position of every image – knowing that usually we are even better than that!

Camera – The Optical Measuring Device

The determination of the camera’s focal point, which in the photogrammetric context is also  referred to as the external orientation only the first part needed for precise mapping has been accomplished. The second equally important need is the so called internal orientation and just describes the optical properties of the camera system used. And by far not every camera is suited for accurate mapping furthermore, most cameras can’t be used for this!

  • Mechanical Shutter – Global Shutter

A basic prerequisite for suitable aerial mapping camera is the shutter system. While most digital camera systems have a rolling shutter, where the sensor read out is done line by line, we need cameras which have a global / mechanical shutter, which is almost unchanged form the shutter systems used in analogue photography. This shutter system ensures a simultaneous exposure and data recording of the entire sensor.

In order to use such a camera system for aerial surveying it needs to be calibrated, meaning that the optical parameters of the camera and lens have to be determined in order to make it available as a measuring device. There is a strong need for constant and reproducible properties! The fewer mechanical parts and movement within the system, the better! For this reason e.g. zoom lenses are highly unsuitable for our needs and the the highly precise methodology.

  • Fokal Speed and Memory Writing Speed Speichergeschwindigkeit

Even with high ground resolution (GSD) and short exposure times which we need for a good positioning accuracy of the focal point relatively high speeds for rotor drones would be acceptable. While flying at 10m/sec a GSD of 10 mm is not a contradiction. However, besides the focus speed – not all cameras allow aerial operations with a reliable preset manual focus – the speed of the data writing to the memory (minly SD-cards) seem to be the most limiting factor for the flight speed.

We are looking forward to you interest on our  aerial mapping capability or for the KLAUPPK precise positioning technology! We are quite confident that we have a solution for your request! Please  call us @ +49 (0)8104 8889810 at any time or send an email  to info@videographics.de!