Network Of Ground Control Points Research Articles

Application of Postprocessing Kinematic Methods with UAS Remote Sensing in Forest Ecosystems

AbstractUnmanned Aerial Systems (UAS) serve as an excellent remote-sensing platform to fulfill an aerial imagery data collection niche previously unattainable in forestry by satellites and manned aircraft. However, for UAS-derived data to be spatially representative, a precise network of ground control points (GCP) is often required, which can be tedious and limit the logistical benefits of UAS rapid deployment capabilities, especially in densely forested areas. Therefore, methods for efficient data collection without GCPs are highly desired in UAS remote sensing. Here, we demonstrate the use of postprocessing kinematic (PPK) technology to obtain subcentimeter precision in datasets of forested areas without the need for placing GCPs. We evaluated two key measures, positional variability and time efficiency, of the PPK technology by comparing them to traditional GCP methods. Results show that PPK displays consistently higher positional precision than traditional GCP approaches. Moreover, PPK surveys and processing take less time to complete than traditional GCP methods and require fewer logistical steps, especially in image acquisition. The time and resource savings with PPK as compared to GCP processing are undeniable. We conclude that PPK technology provides a practical means to produce precise aerial forest surveys.

High-accuracy UAV photogrammetry of ice sheet dynamics with no ground control

Abstract. Unmanned aerial vehicles (UAVs) and structure from motion with multi-view stereo (SfM–MVS) photogrammetry are increasingly common tools for geoscience applications, but final product accuracy can be significantly diminished in the absence of a dense and well-distributed network of ground control points (GCPs). This is problematic in inaccessible or hazardous field environments, including highly crevassed glaciers, where implementing suitable GCP networks would be logistically difficult if not impossible. To overcome this challenge, we present an alternative geolocation approach known as GNSS-supported aerial triangulation (GNSS-AT). Here, an on-board carrier-phase GNSS receiver is used to determine the location of photo acquisitions using kinematic differential carrier-phase positioning. The camera positions can be used as the geospatial input to the photogrammetry process. We describe the implementation of this method in a low-cost, custom-built UAV and apply the method in a glaciological setting at Store Glacier in western Greenland. We validate the technique at the calving front, achieving topographic uncertainties of ±0.12 m horizontally (∼1.1× the ground sampling distance) and ±0.14 m vertically (∼1.3× the ground sampling distance), when flying at an altitude of ∼ 450 m above ground level. This compares favourably with previous GCP-derived uncertainties in glacial environments and allows us to apply the SfM–MVS photogrammetry at an inland study site where ice flows at 2 m day−1 and stable ground control is not available. Here, we were able to produce, without the use of GCPs, the first UAV-derived velocity fields of an ice sheet interior. Given the growing use of UAVs and SfM–MVS in glaciology and the geosciences, GNSS-AT will be of interest to those wishing to use UAV photogrammetry to obtain high-precision measurements of topographic change in contexts where GCP collection is logistically constrained.

Object-Based Classification of Urban Distinct Sub-Elements Using High Spatial Resolution Orthoimages and DSM Layers

This paper aims to assess the ways in which multi-resolution object-based classification methods can be used to group urban environments made up of a mixture of buildings, sub-elements such as car parks, roads, shades and pavements and foliage such as grass and trees. This involves using both unmanned aerial vehicles (UAVs) which provide high-resolution mosaic Orthoimages and generate a Digital Surface Model (DSM). For the study area chosen for this paper, 400 Orthoimages with a spatial resolution of 7 cm each were used to build the Orthoimages and DSM, which were georeferenced using well distributed network of ground control points (GCPs) of 12 reference points (RMSE = 8 cm). As these were combined with onboard RTK-GNSS-enabled 2-frequency receivers, they were able to provide absolute block orientation which had a similar accuracy range if the data had been collected by traditional indirect sensor orientation. Traditional indirect sensor orientation involves the GNSS receiver in the UAV receiving a differential signal from the base station through a communication link. This allows for the precise position of the UAV to be established, as the RTK uses correction, allowing position, velocity, altitude and heading to tracked, as well as the measurement of raw sensor data. By assessing the results of the confusion matrices, it can be seen that the overall accuracy of the object-oriented classification was 84.37%. This has an overall Kappa of 0.74 and the data that had poor classification accuracy included shade, parking lots and concrete pavements. These had a producer accuracy (precision) of 81%, 74% and 74% respectively, while lakes and solar panels each scored 100% in comparison, meaning that they had good classification accuracy.

Unmanned aerial vehicle based remote sensing method for monitoring a steep mountainous slope in the Three Gorges Reservoir, China

Monitoring has been considered to be the most effective means of preventing and mitigating geo-hazards. Unmanned aerial vehicle (UAV) based remote sensing has been proven to be a viable method for monitoring relatively flat areas, but has typically encountered challenges on steep mountainous slopes. The Qinglingou Slope, which is located at the head of the Three Gorges Reservoir, China, was selected as a case study, and a UAV-based remote sensing method was applied to monitor this steep mountainous slope. The UAV, which was equipped with a camera, was used to acquire images on May 10 and Sept. 13, 2015. At the same time, a plan for establishing and measuring a detailed network of ground control points (GCPs) and check points was carefully designed and implemented, according to the local conditions. Georeferenced digital surface models (DSMs) and digital orthophotos were obtained by photogrammetric processing. Based on the results, a simple and quick method that identified and quantified slope deformation by calculating and analyzing the changes in the DSMs was developed. The results showed that the UAV-based remote sensing method was rapid, safe and effective, and it could be valuable for monitoring a large number of steep mountainous slopes in the Three Gorges Reservoir.

Spatial Accuracy of UAV-Derived Orthoimagery and Topography: Comparing Photogrammetric Models Processed with Direct Geo-Referencing and Ground Control Points

Mapping with unmanned aerial vehicles (UAVs) typically involves the deployment of ground control points (GCPs) to georeference the images and topographic model. An alternative approach is direct geo ref er encing, whereby the onboard Global Navigation Satellite System (GNSS) and inertial measurement unit are used without GCPs to locate and orient the data. This study compares the spatial accuracy of these approaches using two nearly identical UAVs. The onboard GNSS is the one difference between them, as one vehicle uses a survey-grade GNSS/RTK receiver (RTK UAV), while the other uses a lower-grade GPS receiver (non-RTK UAV). Field testing was performed at a gravel pit, with all ground measurements and aerial sur vey ing completed on the same day. Three sets of orthoimages and DSMs were produced for comparing spa tial accuracies: two sets were created by direct georeferencing images from the RTK UAV and non-RTK UAV and one set was created by using GCPs during the external orientation of the non-RTK UAV images. Spatial accuracy was determined from the horizontal (X,Y) and vertical (Z) residuals and root-mean-square-errors (RMSE) relative to 17 horizontal and 180 vertical check points measured with a GNSS/RTK base station and rover. For the two direct georeferencing datasets, the horizontal and vertical accuracy improved substantially with the survey-grade GNSS/RTK receiver onboard the RTK UAV, effectively reducing the RMSE values in X, Y and Z by 1 to 2 orders of magnitude compared to the lower grade GPS receiver onboard the non-RTK UAV. Importantly, the horizontal accuracy of the RTK UAV data processed via direct georeferencing was equivalent to the horizontal accuracy of the non-RTK UAV data processed with GCPs, but the vertical error of the DSM from the RTK UAV data was 2 to 3 times greater than the DSM from the non-RTK data with GCPs. Overall, results suggest that direct georeferencing with the RTK UAV can achieve horizontal accuracy comparable to that obtained with a network of GCPs, but for topographic meas urements requiring the highest achievable accuracy, researchers and practitioners should use GCPs.

Influence of surface height variance on distribution of ground control points

The process of geometric correction of a satellite image requires a network of ground control points (GCPs), the density of which depends on the accuracy required. In this study, an approach to determine the distribution of the GCPs was developed based on dividing the surface terrain into zones of low- and high-height variances. Comparative statistics were investigated to summarize the optimum number and distribution of the GCPs. For a uniform arrangement of 10 GCPs, the accuracy using root mean square error was 7.19 m. This accuracy was improved to 6.13 m following the inclusion of just two additional GCPs in the zone of high-surface height variance. Thus, the transformation using the polynomial model and a set of GCPs, for which the surface variance in height was considered, resulted in greater accuracy than using the conven- tional uniformly distributed method. In addition, the time required for the trial-and-error selec- tion of the locations of the GCPs was reduced. Our results suggest that a method that considers the variance in height of the surface terrain could be applied to various types of images such as satellite or aerial photography. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10.1117/1.JRS.8.083684)

Can old analogue sidescan sonar data still be useful? An example of a sonograph mosaic geo-coded by the DECCA navigation system

In the Baltic and the North Sea area the collection of sidescan sonar data began in the early 1970s. Although positioning systems at that time were much less accurate than today, such analogue data archives are a valuable source of comparison for estimating spatial and temporal changes in sediment patterns. This paper describes how geographic information systems (GIS) and remote sensing methods are being applied for more accurate positioning and geometric correction of analogue sidescan sonar profiles geo-referenced by the DECCA navigation system (DNS). A 20 km 2 area in the North Sea, where even after 25 years a complicated sediment pattern has only changed slightly, was selected for the comparison. Data from two sidescan sonar surveys, one from 1977 and the other from 2002, were available. The 2002 data, acquired with a positioning accuracy of estimated 25 m, were mosaicked using modern digital processing techniques. This dataset was used to estimate positioning errors in the analogue sidescan sonar profiles of 1977. After the application of a geo-referencing method of ‘rubber sheeting’ based on an irregular network of ground control points to generate a mosaic of the analogue profiles, positioning errors of 211 m in the E–W and 98 m in the N–S directions were obtained. The positioning error of the DNS calculated from equations provided by the DECCAN Navigator Company for this area, was as large as 200 m for the ‘purple’ station transmission, 197 m for the ‘green’, and 66 m for the ‘red’ one. Consequently, we conclude that the observed positioning errors were mainly caused by systematic deviations of the DNS rather than simplifications used in our methodology. Since such large errors can be corrected locally with some confidence, analogue sidescan profiles geo-referenced by DNS can be used as a significant source of information after digital image processing at appropriate map scales.

Achieving sub-pixel geolocation accuracy in support of MODIS land science

The Moderate Resolution Imaging Spectroradiometer (MODIS) was launched in December 1999 on the polar orbiting Terra spacecraft and since February 2000 has been acquiring daily global data in 36 spectral bands—29 with 1 km, five with 500 m, and two with 250 m nadir pixel dimensions. The Terra satellite has on-board exterior orientation (position and attitude) measurement systems designed to enable geolocation of MODIS data to approximately 150 m (1 σ) at nadir. A global network of ground control points is being used to determine biases and trends in the sensor orientation. Biases have been removed by updating models of the spacecraft and instrument orientation in the MODIS geolocation software several times since launch and have improved the MODIS geolocation to approximately 50 m (1 σ) at nadir. This paper overviews the geolocation approach, summarizes the first year of geolocation analysis, and overviews future work. The approach allows an operational characterization of the MODIS geolocation errors and enables individual MODIS observations to be geolocated to the sub-pixel accuracies required for terrestrial global change applications.

Low altitude, small format aerial photogrammetry

Abstract This project describes the use of 35 mm format non-metric photography for estimating earth work. The aerial pictures were taken from 35 mm super high speed, non-metric cameras mounted on remote-controlled model aeroplane. Results from a test-bed using a network of ground control points suggest that the accuracy of obtaining X, Y, Z ground co-ordinates from digitized enlarged positive prints using desk-top digitizers was adequate for most engineering work.