Abstract: This article presents the results of an assessment of radiometric correction methods of images taken by the large-format aerial, photogrammetric, multispectral pushbroom camera Leica Geosystems ADS40. The investigation was carried out in the context of the multi-site EuroSDR project “Radiometric aspects of digital photogrammetric images”. Images were collected at the forestry research test site Hyytiälä, Finland in August, 2008. Two processing workflows were evaluated: one based on the photogrammetric software Leica XPro, which in radiometric processes relies on physical modeling and information collected from the imagery only, and one based on ATCOR-4, which is software dedicated to physical atmospheric correction of airborne multi-, hyperspectral and thermal scanner data, and can be operated either with or without in-situ reflectance and atmospheric observations. Outputs of these processes are reflectance images. Three participants processed the data with several processing options which resulted in a total of 12 different radiometrically corrected reflectance images. The data analysis was based on field and laboratory reflectance measurements of reference reflectance targets and field measurements of permanent targets (asphalt, grass, gravel). Leica XPro provided up to 5 % reflectance accuracy without any ground reference and ATCOR-4 provided reflectance accuracy better than 5 % with vicarious in-flight radiometric calibration of the sensor. The results show that the radiometric correction of multispectral aerial images is possible in an efficient way in the photogrammetric production environment.
Abstract: Solar elevation is an important factor in passive, airborne data collection. The minimum solar elevation
allowed in missions for topographic mapping is typically 30° from the horizon. A general hypothesis is
that the new, high dynamic range, digital large-format photogrammetric sensors allow for high data
quality, even with lower solar elevations, which would improve the feasibility and cost-efficiency of pho-
togrammetric technology in various applications. Objectives of this study were to investigate theoreti-
cally and empirically the impacts of solar elevation in modern photogrammetric processes. Two
cutting-edge aspects of novel photogrammetric technology were considered: point cloud creation by
automatic image matching and reflectance calibration of image data. For the empirical study, we used
image data collected by a large-format photogrammetric camera, Intergraph DMC, with low (25–28°)
and medium (44–48°) solar elevations from 2, 3 and 4 km heights. We did not detect negative influences
of decreasing solar elevation during our general evaluations: an analysis of image histograms showed
that the ranges of digital numbers could be tuned to similar levels with exposure settings, and evalua-
tions of density and the accuracy of point clouds did not show any reduction of quality. We carried
out detailed evaluations in forests, roads and fields. Our results did not indicate deterioration of the qual-
ity in sun-illuminated areas with decreasing solar elevation. In shadowed areas, we observed that the
variation of image signal was reduced in comparison to sun-illuminated areas and emphasized the issue
of complication of reflectance calibration. Artefacts appeared in automatically generated point clouds in
areas shadowed by trees, which we observed in flat objects as up to 3 times increased random height var-
iation and decreased success in measuring the terrain surface. Our results also showed that the overall
performance of point cloud generation was high. Typically, point clouds could be derived even from a sin-
gle stereo model with the point density corresponding to the GSD, but some expected and unexpected
failures also appeared. The height accuracy was dependent on the object properties and the intersection
geometry; the height accuracy was 0.5–2 times GSD at well defined objects. Our conclusions were that in
the future it is of increasing importance to quantify the sensitivity of different methods on the radiomet-
ric properties of the image data. It is also important to develop interpretation methods that are not sen-
sitive to shadows, in order to enable optimal use of photogrammetric technology in normal to rapid
response applications.
Abstract: The objective of this investigation is to develop and test a radiometric correction process for UAV image blocks. Phases of the process are the laboratory calibration of the sensor and the radiometric correction of the campaign image data. This investigation focuses on the development of a process for radiometric correction of the image data collected in the remote sensing campaign. First of all, the orientations of the images are determined using the self-calibrating bundle block adjustment method and an accurate DSM is generated by automatic image matching. The varying radiometric level of images due to changes in illumination and instability of the sensor are eliminated using a relative radiometric block adjustment technique. Optional reflectance reference observations can be used to adjust the data to absolute reflectance units. The process was demonstrated and evaluated by using two UAV imaging systems: a consumer camera based system and a novel Fabry-Perot interferometer based next generation light weight hyperspectral imaging system. The method improved homogeneity of the data, but also some drift appeared in the parameters. The first experiment provided 0.003-0.008 reflectance errors in closure to the control points (mostly on the level of 5% of the reflectance value). The presented approach is a general framework for rigorous radiometric correction of UAV image blocks, and the novel technology provides many possibilities for the further development of the method. Our results also showed that hyperspectral stereophotogrammetry is now possible with light weight UAV sensors, weighting less than 500 g.
Abstract: Novel photogrammetric sensors open up new possibilities for measuring the radiometric and geometric properties of the earth’s surface. To ensure optimal use of these complex, rapidly developing, diverse systems, it is becoming crucial to have a means of calibrating and validating complete imaging systems, and even production lines, in operational conditions. This paper presents a calibration and validation methodology based on a permanent test site (Sjökulla) and demonstrates its use in a calibration and validation campaign for a large-format photogrammetric imaging system, the Intergraph DMC. The major objective of the campaign was to perform acceptance testing of the new camera system procured by the National Land Survey of Finland. The evaluation proved the high geometric and radiometric performance of the imaging system and demonstrated the potential of spectrodirectional photogrammetric 3D imaging. The usability aspects of test site calibration and validation from the point of view of a mapping organisation were also considered
Abstract: Ilmakuvaus on ollut jo pitkään keskeinen menetelmä karttatiedon tuottamiseen. Uudet digitaaliset kuvausjärjestelmät tuottavat tehokkaasti ja nopeasti ajantasaista kuvatietoa paikkatietojärjestelmiin. Uudet automaattiset prosessointimenetelmät, erityisesti kuvien sävyarvojen reflektanssikalibrointi ja kuvansovituksen avulla tuotettavat pistepilvet, avaavat entistä parempia mahdollisuuksia automatisoida kuvatukintaprosesseja. Ilmakuvia voidaan käyttää erilaisissa kaukokartoitussovellutuksissa sellaisenaan tai yhdistämällä muihin kaukokartoitusaineistoihin, kuten laserkeilaukseen. Artikkelin tavoitteena on selvittää modernin ilmakuvauksen ominaisuuksia sekä luoda katsaus tulevaisuuteen viimeaikaisten tutkimustulosten perusteella.
Abstract: Radiometric stability is a desired property of digital photogrammetric large-format sensors. This article
presents a methodology for determining the radiometric stability of airborne imaging sensors in operational
conditions in a test field and the results of stability evaluation of a large-format photogrammetric
frame sensor DMC, from Intergraph. The imagery was collected in two days using nine different exposure
settings, and images collected with variable exposure time and aperture were compared. The results
showed promising stability in many cases, up to a level of 2% of the radiance, but less favorable results also
appeared. Possible reasons for the unfavorable results could be the limitations of the experimental set-up
or the instability of the sensor. DMC showed high radiometric performance potential, but high sensitivity
to the exposure settings. Based on the results, recommendations for the future test field calibration and
validation procedures were given. One limitation of the analysis was the insufficient information about
the sensor stability potential; proposals were given to sensor manufacturers concerning the necessary
information.
Abstract: Large-format photogrammetric digital airborne imaging sensors have been commercially available for several years. Their excellent radiometric properties compared to film-based imaging have been reported in several studies. Despite these radiometric advantages, up to now, airborne digital images have been exploited in a rather conventional manner in photogrammetric applications, even though methodologies from quantitative remote sensing, e.g. radiometric image correction and classification, could be utilized to raise the performance of photogrammetric applications to a new level. The recent state-of-the-art review revealed that the fundamental problem in the quantitative utilization of image radiometry in photogrammetric applications is the radiometric correction. Among digital photogrammetric large-format mapping sensors, the Airborne Digital Sensor (ADS) of Leica Geosystems is the only commercially available system at the moment having an integrated, physically based, radiometric correction chain. The processing does not require any in situ control information, as the radiometric corrections are based on a priori calibration information and image data. To validate performance of the processing chain, a comprehensive flight campaign was carried out with an ADS40 SH52 sensor in Finland in August 2008. We present the first results of the validation of the Leica ADS40 radiometric processing chain. The results indicated great performance potential. With the challenging data set, the differences of ADS40 and independent ground reference reflectance measurements were even less than 5% for uniform targets; atmospheric state, multispectral channel and flying height were detected as the major factors influencing the accuracy. Leica Geosystem’s ADS40 can be considered as an efficient and accurate, 3D, multi-angular, multispectral imaging radiometer, which opens new interesting prospects for 3D remote sensing and characterization of the Earth surface. Results also indicated the importance of the test field validation process, gave improvement ideas for the sensor post-processing software and provided information for the development of validation methods.
Abstract: There is currently high interest in developing automated methods to assist the updating of map databases. This study presents methods for automatic detection of buildings and changes in buildings from airborne laser scanner and digital aerial image data and shows the potential usefulness of the methods with thorough experiments in a 5 km2 suburban study area. 96% of buildings larger than 60 m2 were correctly detected in the building detection. The completeness and correctness of the change detection for buildings larger than 60 m2 were about 85% (including five classes). Most of the errors occurred in small or otherwise problematic buildings.
Abstract: Comprehensive field-testing and calibration of digital photogrammetric systems are essential to characterize their performance, to improve them, and to be able to use them for optimal results. The radiometric, spectral, spatial, and geometric properties of digital systems require calibration and testing.
The Finnish Geodetic Institute has maintained a permanent test field for geometric, radiometric, and spatial resolution calibration and testing of high-resolution airborne and satellite imaging systems in Sjökulla since 1994. The special features of this test field are permanent resolution and reflectance targets made of gravel. The Sjökulla test field with some supplementary targets is a prototype for a future photogrammetric field calibration site.
This article describes the Sjökulla test field and its construction and spectral properties. It goes on to discuss targets and methods for system testing and calibration, and highlights the calibration and testing of digital photogrammetric systems.
Abstract: Test field calibration is an attractive approach to calibrating and characterizing the radiometry of airborne imaging instruments. In this study, a method for radiometric test field calibration for digital photogrammetric instruments is developed and it is used to evaluate the radiometric performance of large-format photogrammetric sensors the ADS40, the DMC, and
the UltraCamD. In the study, linearity, dynamic range, sensitivity, and absolute calibration were evaluated. The results demonstrated the high radiometric quality of the sensors tested. All the sensors were linear in response. The DMC used the 12-bit dynamic range entirely, while the ADS40 and the UltraCamD indicated close to the 13-bit dynamic range. The sensors performed quite differently with respect to sensitivity. With the DMC and the UltraCamD a risk of overexposure appeared, while the color channels of the ADS40 showed low sensitivity. Because the sensors were linear in response, they could be absolutely calibrated using linear models.
Abstract: This article presents the empirical research carried out in the context of the multi-site EuroSDR project “Radiometric aspects of digital photogrammetric images” and provides highlights of the results. The investigations have considered the vicarious radiometric and spatial resolution validation and calibration of the sensor system, radiometric processing of the image blocks either by performing relative radiometric block equalization or into absolutely reflectance calibrated products, and finally aspects of practical applications on NDVI layer generation and tree species classification. The data sets were provided by Leica Geosystems ADS40 and Intergraph DMC and the participants represented stakeholders in National Mapping Authorities, software development and research. The investigations proved the stability and quality of evaluated imaging systems with respect to radiometry and optical system. The first new-generation methods for reflectance calibration and equalization of photogrammetric image block data provided promising accuracy and were also functional from the productivity and usability points of view. The reflectance calibration methods provided up to 5% accuracy without any ground reference. Application oriented results indicated that automatic interpretation methods will benefit from the optimal use of radiometrically accurate multi-view photogrammetric imagery.
Abstract: The use of digital aerial images within goverment, municipalities, research institutes and private companies is increasing rapidly. At the moment, most of the images are taken on traditional film, but digital aerial cameras are getting more common. Major users of aerial images in Finland are forest research institutes and Ministry of Agriculture and Foresty (MAF). Obliged by European Union, MAF maintains the Land Parcel Identification System.
In order to use film images on computers, they must be digitized with photogrammetric scanner. Geometric accuracy of photogrammetric scanners is known and handled well, whereas there is no method for the quality control for the radiometric properties of scanners.
In order to use same images both in land parcel identification and forest interpretation, MAF has chosen to use close infrared film (CIR), which records close infrared, red and green wavelengths. Forest interpretation is based on tonally adjusted CIR images, from where one can separe coniferous and deciduous tree species. Guidelines for these adjustments have been missing or the adjustments are based on subjective criteria.
In this work, quality control system for scanned aerial images based on image histograms was created. The foundation of this study were 4609 image histograms from two finnish image producers. Histograms were collected with software called HISTOQC, coded at Finnish Geodetic Institute. The most important histogram statistics were found to be 100%- and 99%-efficiency and 0% and 255%-saturation. Acceptance and recommended criteria for these statistics are given.
In the second part of this work, tonally adjusted model-image for forest interpretation was created. Model-image was based on test adjustments and ground control with forest interpretation specialists. Goals of the image adjustements were following: 1) separate coniferous and deciduous trees 2) allow forest species intrerpretation from mixed forests 3) find a separe tone for non-organic targets 4) avoid saturation of dark parts of the fields.
The developed method for quality control of scanned images has been taken in use in the scanning processes of finnish image producers. The created model-image was delivered to image producers and forest research institutes.
Notes:
Mat-1.125 Special course assignment in mathematics
