15arspc submission 150

8/8/2019 15arspc Submission 150

1/12


2/12


3/12

3

1.0 IntroductionThe New South Wales Land and Property Management Authority (LPMA) has,since 1947, routinely captured aerial imagery of the State s landscape throughfilm based photography and, from 2007, digital imagery utilising the LeicaGeosystems ADS40 sensor. From 1997 onwards, this imagery has beenregularly held in digital orthorectified form, facilitating its use in a GeographicInformation System (GIS) alongside other digital data.

The acquisition and processing of such aerial imagery has proven to be anessential element in decision making and planning processes for variousgovernment agencies, particularly those charged with the responsibility ofresponding to natural and man-made emergencies such as flooding, severestorms and environmental events.

Due to the increase in demand for aerial imagery covering such events, theLPMA imagery team has developed processes to implement a rapid responseimagery capability delivering quality products and services in a timely and costefficient manner for their clients. This paper covers the different processes usedto produce Rapid Response imagery, in contrast to LPMAs standard productionimagery

2.0 BackgroundThrough its standard imagery capture program, LPMA has officially recordedthe States development as a key component of the NSW Spatial DataInfrastructure (SDI). The SDI comprises a suite of initiatives to ensure NSWskey spatial datasets are current, comprehensive, accurate and readily available

for the benefit of government and other geospatial users.A key component of the LPMA Imagery capability is the Leica GeosystemsADS40 sensor used for capturing the imagery. The ADS40 utilises a continuouspush broom scanning process to record imagery in 3 look angles; forward, nadirand backward views. The ADS40 is a multi-spectral sensor that capturespanchromatic bands in the forward, nadir and backward, angles, along withRed, Green, Blue (RGB) and Near Infrared bands (IR) in the nadir andbackward look angles (Bachofen et al . 2008). The multi-spectral nature of theADS40 has opened further analysis capabilities, including image classification.

The design of the ADS40 and associated software allows for the direct

georeferencing of the acquired image data without the use of ground controlpoints (Casella et al . 2008). LPMA and its clients have found this approachadequate for Rapid Response Imagery applications, where time becomes thecritical factor for decision makers.

The successful implementation of the ADS40 has lead to improvements andefficiencies in the acquisition and processing of airborne data for both standardand rapid response imagery production.

3.0 Standard Production Imagery vs Rapid Response ImageryFor LPMA, standard production refers to the systematic acquisition andprocessing of imagery over a 1:100,000 mapping block made up of


4/12

4

approximately 12 flight lines or strips and based on the standard NSWTopographic Map Series.

Standard production imagery is flown according to a systematic programmeestablished in consultation with government agencies and other users. Theimagery has a nominal Ground Sampling Distance (GSD) of 50cm and a revisitperiod of 3-5 years over eastern and central NSW. Standard production imageryacquisition is bound by National Mapping standards covering such variables ascloud cover, atmospheric conditions and solar altitude limits to ensure optimumconditions for capture and acceptable accuracy of processed image data(National Mapping Council of Australia 1985).

In contrast, rapid response imagery projects are ad hoc by definition and areundertaken at short notice. Acquisition is planned in consultation withemergency services and government agencies with a view to meeting theneeds of the client for each unique event. Due to the time-critical nature ofcapture and processing, rapid response imagery standards are necessarilymore relaxed, with imagery often flown outside normal solar altitude limits orunder cloudy conditions.

4.0 Airborne GNSS processing for Rapid Response ImageryGlobal Navigation Satellite System (GNSS) data is recorded in conjunction withthe imagery during an aerial survey flight. This data is processed to provideaccurate positional information for aerial triangulation. There are two differentmethods of processing GNSS data; the traditional method of Differential GNSS(DGNSS) positioning utilising a base station or the use of Precise PointPositioning (PPP). Both methods have a number of advantages anddisadvantages when processing GNSS data for rapid response projects.

Differential GNSS positioning requires that one or more base stations aresimultaneously recording data on the ground, while the airborne GNSS receiveris recording data during the aerial survey flight (Yuan, et al., 2009). In order toachieve the best results from this method the base station should be locatedclose to the project area; preferably within 50 kilometres (Leica Geosystems2007). With LPMAs CORSnet -NSW network still being rolled out across theState in 2010, the location of these base stations is typically not favourable, andthe option of setting a temporary base station in an area not serviced by a basestation is generally not considered practical because of time issues.

Precise Point Positioning is a GNSS processing technique in which only asingle GNSS receiver located on the aircraft is required. PPP utilises preciseGPS satellite ephemeris parameters and clock error correction products fromthe International GNSS Service (IGS) to process the airborne GNSS data (Yuanet al . 2009). The primary advantage of using PPP is that it is capable ofproducing centimetre level accuracy for kinematic GNSS data, without requiringa base station (Yuan et al . 2009). The perceived drawback to the use of PPP forrapid response imagery is the reduced accuracy of the precise clock andephemeris products if used within 13 days of the airborne GNSS data beingrecorded. Table 1 outlines the relative accuracies of the various IGS precise

clock and ephemeris products.


5/12

5

Table 1. Precise Point Positioning ephemeris and clockproduct accuracies (International GNSS Service, 2009).

Product Accuracy LatencyOrbit (cm) Clock (ns)IGS Ultra Rapid

(estimated) ~3 ~0.2 3 hours

IGS Rapid ~2.5 0.1 17 hoursIGS Final


6/12

6

Seamlines are required for mosaicing (joining) each of the flight lines in aproject. The seamlines created for all jobs are stored in a geodatabase. As aresult, these seamlines can be reused for rapid response imagery should thesame flight lines be flown as a previous job. Potentially, this can reduce manual

editing time from 3-4 days to several hours.The most time consuming part of the orthophoto production process is that ofsurface model creation. Each surface model is generated from ADS40 stereo-imagery utilising auto-correlation techniques, often taking up to a week tocreate and requiring further manual editing. The generation of a surface modelis not feasible for rapid response imagery due to the time constraints involved.This is overcome by the utilisation of existing ADS40 derived surface modelswhere available or acquiring from other sources, such as the coarse 25 metrestate-wide surface model held by LPMA.

This use of previously created data becomes a significant time saving techniquethat is used to its full potential to expedite the production of rapid responseimagery jobs.

6.0 Colour Infrared ImageryThe ADS40 is a multi-spectral sensor that captures red, green, blue (RGB) andNear Infrared bands in the backward and nadir look angles. Colour infrared(CIR) imagery consists of green, red and near infrared bands.

Figure 1: Wavelengths of the ADS40 spectral bands (Wagner 2008).

The production of CIR ortho-rectified imagery in rapid response imagery iscrucial for further image analysis. Different ground cover features reflect varyingamounts of Near Infrared (NIR) energy. Healthy green vegetation has a highNIR reflectance. Water and asphalt, in particular, absorb almost all of the NIRwavelengths of light, allowing these features to be easily identified. This alsomakes flood water identification more apparent as it will vary from black to light

shades of blue depending on the level of sedimentation. Figure 2 highlights


7/12

7

these differences in feature reflectance values when displayed in false colour(Figure 2 a ) and true colour imagery (Figure 2 b )

a b

Figure 2: Penrith Regatta Centre. a : False Colour (CIR). b : True Colour (RGB)

7.0 Classification of Flood ImageryThe multi-spectral capability of the ADS40 sensor enables accurate extractionof different land-cover classes through classification of the imagery.

In response to flood events in 2010, inundating vast regions of north-west NewSouth Wales, LPMA was requested to capture imagery of flood waters in the

Paroo Overflow to the north of Wilcannia. Figure 3 shows the extent of theimagery captured for this project in both false colour Infrared (Figure 3 a ) andtrue colour (Figure 3 b ).

Using 4-band imagery consisting of blue, green, red and near infrared bands, asupervised classification was undertaken to extract flood water areas. Spectralsignatures were collected for a small number of ground-cover types including;water, bare soil and vegetation. Utilising the band combinations available, theseground-cover classes were easily identifiable. Had ground-truth data beenavailable, the accuracy of the classification would have been further improved.However given the nature of the job and the priority being placed on classifyingthe flooded regions, this lack of ground-truth data did not adversely affect theoverall classification result.

The results of the classification can be seen in Figure 4( a ). The extent of theflood waters has been accurately classified, while the vegetation and soilclasses have also been well delineated. Figure 4( b ) shows the water class as apolygon shapefile. This layer can then be used for further analysis inconjunction with other datasets.

Results show that it is possible to undertake a straight-forward supervisedclassification using 4-band ADS40 imagery that accurately identifies a numberof basic land-cover classes. Due to the time saved in the production of rapidreponse imagery, additional analysis such as this can be undertaken to furtherassist emergency services response to the flood event.


8/12

8

a b Figure 3: Paroo Overflow Flood area. a : False Colour (CIR). b : True Colour (RGB).

a b Figure 4: a : Result of supervised classification of Paroo Overflow

flood imagery, showing water, soil and vegetation classes. b : Water extent extracted

from the classified image and converted to vector polygon.


9/12

9

8.0 Examples of Rapid Response Imagery Projects

8.1 NSW Riverina Red Gum Research

Figure 5: New South Wales Riverina region Red Gum research area coverage(Background 1:100,000 mapping sheets)

Due to environmental events occurring in the Riverina region in 2010, LPMAwas tasked to fly a number of specialised areas for Red Gum research at 50cmGSD. The area, covering almost 5700km 2, consisted of a number of separateblocks and single flight lines, see Figure 5. The unavailability of ground controlpoints for these areas meant that the accuracy of the aerial triangulationprocess relied on Rapid PPP data. Further, LPMAs 25m state -wide DTM wasused as no surface models from previous jobs were available.

In total 11 areas were covered, with 11 RGB mosaics, 11 CIR mosaics andmultiple 4 band imagery tiles created. This enormous task was processed bythe LPMA team in 4 days, demonstrating LPMAs rapid response imagerycapabilities.

8.2 North-west NSW FloodIn early 2010, over a period of 4 weeks, large regions of North-west New SouthWales were inundated with flood waters (see Figure 6). In response, emergencyservices requested that LPMA capture imagery of the flooded areas. Thefollowing blocks, totalling an area of 18000km 2, were flown:


10/12

10

Brewarrina 1:100,000 RGB and CIR 50cmBourke 1:100,000 RGB and CIR 50cmNarran 1:100,000 RGB and CIR 50cmCarinda 1:100,000 RGB and CIR 50cm

Walgett 1:100,000 RGB and CIR 50cmToorale Part 1:100,000 RGB and CIR 30cmLouth Part 1:100,000 RGB and CIR 50cmCumborah Part 1:100,000 RGB and CIR 50cmGeera Part 1:100,000 RGB and CIR 50cmCoonamble Part 1:100,000 RGB and CIR 50cmCoonamble Town RGB and CIR 10cm

Figure 6: North-west New South Wales Flood coverage (Background 1:100,000

mapping sheets)These particular areas were processed using Rapid PPP due to the urgency ofthe imagery. Previously derived seamlines, control points and surface modelswere used where available, as some of the areas of interest had already beencaptured under LPMAs Standard I magery Program. The reusability of this datasaved weeks of processing and manual editing.

Within a time span of 5 weeks from the first flight date, the LPMA teammanaged to deliver 22 final products, including individual RGB and ColourInfrared Mosaics. Again, this demonstrates LPMAs rapid response imagerycapabilities.


11/12

11

9.0 ConclusionWith the introduction of the ADS40 digital sensor in 2007, LPMA was able tosubstantially reduce the amount of processing time required for standardimagery processing compared to film-based image processing. However, withincreasing demand from emergency services and government agencies for arapid supply of high quality imagery in times of natural disaster or emergency, itbecame clear that the processing time had to decrease further in order to bettermeet these needs.

The LPMA imagery team undertook intensive research into the imageryprocessing workflow in order to determine where time-savings could be made,whilst still providing a high quality product to the client. This research involvedexamining areas of the workflow such as the GNSS processing, where it hasbeen determined that the accuracy achieved using Rapid PPP is suitable. Anumber of geodatabases were also developed to allow for the storage and re-

use of auxiliary data, such as ground control points, surface models andseamlines. This reusability of data has lead to a significant increase in efficiencywhen processing rapid response imagery.

Research into imagery products has revealed that colour infrared imagery isbeneficial for an effective response to the event or disaster. The availability ofCIR products allows for further interpretation of the imagery. This includesclassification of the rapid response imagery, which can be particularly valuablein assessing flood events.

The LPMA imagery applications team have proven that it is capable ofproducing rapid response imagery in a timely manner to provide emergency

services and government agencies with high quality imagery that meets theirneeds and allows the response to emergency situations to be more efficient andeffective.

ReferencesBachofen, D., Kirchhofer, W., Saks, T., Steinmann, P., Sun, H., Vonblon, L.,Wagner, R., Zuberbhler, F., 2008. New Developments on PushbroomSensors. The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences , Vol. XXXVII , Part B1, pp. 688-690.

Casella, V., Franzini, M., Banchini, G., Gentili, G., 2008. Initial Evaluation of theSecond-Generation Leica ADS40 Camera . The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences , Vol. XXXVII, Part B1, pp. 527-532.

International GNSS Service, 2009, IGS Products. Available online at:http://igscb.jpl.nasa.gov/components/prods.html (Viewed 15 June 2010).

Leica Geosystems, 2007, ADS40 Documentation, Volume 3, Field Manual .Version 2.12-86 (Heerbrugg: Leica Geosytems AG).


12/12

12

National Mapping Council of Australia, 1985. Standard Specifications for Black and White Vertical Aerial Photography for Line Map Production . (Canberra:Department of Resources and Energy).

Wagner, R., 2008, Leica ADS80 Digital Airborne Imaging Solution .Presentation at a conference. Leica Geosystems Airborne Sensor Workshop,27 October 2008, San Ramon, CA.

Yuan, X., Fu, J., Sun, H. and Toth, C., 2009, The Application of GPS PrecisePoint Positioning Technology in Aerial Triangulation. ISPRS Journal of Photogrammetry and Remote Sensing , 64 , pp. 541-550.

15arspc submission 150

Documents