15arspc Submission 175

8/8/2019 15arspc Submission 175

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Fly Once, Use Many: Creating Multipurpose Imagery

Leanne Mills

Senior Spatial TechnicianImagery Applications

Land and Property Management Authority346 Panorama Avenue, Bathurst NSW 2795

Ph: +61 2 6332 [email protected]

Abstract

Public awareness of aerial imagery has dramatically increased over the past 10years. Applications of imagery is stepping out of traditional uses such astopographic mapping and moving towards other realms; specifically emergencyservices, environmental applications, agricultural monitoring and lawenforcement. The need to produce a quality high resolution product that can beused for many applications is now in high demand.

The Land and Property Management Authority (formerly New South WalesDepartment of Lands) has been capturing aerial imagery of New South Walessince 1947. In 2007, LPMA replaced the film-based system with an ADS40SH52 sensor from Leica Geosystems. The ADS40 is a pushbroom style sensorwith the ability to simultaneously capture twelve bands of data in three differentlook angles. The sensor can produce a number of products from the onecapture. These products include standard colour, false colour, 8bit, 16bit and

multiband imagery. Since the acquisition of the sensor, both the AircraftOperations and Imagery Application teams within LPMA have been identifyingways to streamline the orthophoto production process in a way that doesntcompromise image accuracy or quality. The workflow process has been refinedto nine main steps starting with flight planning through to metadata creation. Akey component of the workflow is saving auxiliary files so they can be used forthe generation of other products as well as the re-processing of areas that havebeen flown again.

This paper will be of interest to organisations that use or are thinking aboutusing imagery for a project. It will describe how LPMA creates orthorectified

imagery from the ADS40 sensor and the different products that can be createdfrom one imagery project.


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Introduction

In 2007, the Land and Property Management Authority decided to replace thefilm based RC30 camera in order to better meet client needs and to become anauthority in orthophoto image production. Leica Geosystems was awarded the

tender and in May 2007, LPMA purchased the ADS40 SH52 sensor, associatedsoftware and hardware. The Digital Image Acquisition System (DIAS) programbegan in June 2007.

Changing sensors and moving from film to digital based photography required acomplete workflow redesign and training of staff in both the Aircraft Operationsand Imagery Applications teams. Due to the unfamiliar nature of the newtechnology, LPMA had to educate clients and stakeholders in the capabilities,terminology and products available from the sensor. Initially the products werebased on those created using the film based imagery. It soon became apparentthat this product list did not reflect the capabilities of the new sensor or the new

requirements of the clients. After much client consultation the standard productlist was changed to reflect new market needs.

ADS40 SH52 Sensor

The ADS40 SH52 is an Airborne Digital Sensor which is designed and suppliedby Leica Geosystems. It captures data in the red, blue, green, near infrared andpanchromatic bands. The camera is a pushbroom style sensor that recordsdata in three different look angles. The backward (16

o) and nadir (0

o) look

angles each capture the red, green, blue and panchromatic bands whilst theforward (27

o) look angle captures one panchromatic band.

Figure 1: Data capture in the ADS40 (Leica Geosystems, 2007)


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LPMA operates the sensor in a Cessna 421c aircraft. The main physicalcomponents of the system consist of: the pilot display, operator interface,sensor head, PAV unit (gyro-stabilised sensor mount), control unit and massmemory.

Figure 2: Data capture in the ADS40 (Leica Geosystems, 2007)

Figure 3: The ADS40 configuration inside the aircraft.


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Orthophoto Workflow

An imagery project is first initiated by the Manager of Imagery and ElevationPrograms or either of the Imagery or Aircraft Team Supervisors. The AircraftOperations Team is notified of the project request and begins flight planning to

produce a planned flight date. The Survey Branch is notified of the plannedflight date so control points can be collected before the job is flown. Controlpoints can be either targets or permanent features on the landscape. Theproject is then flown, downloaded and given to the Imagery Applications Teamto process.

Figure 4: LPMA job initiation

The orthophoto production process is divided into twelve main steps. The firstthree steps, flight planning to imagery download, are performed by the AircraftOperations Team. The remaining nine steps, imagery processing to productcreation, are performed by the Imagery Applications Team (refer to Figure 5).Using this workflow, LPMA is able to produce a range of products from oneproject to suit a variety of client needs.


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Figure 5: LPMA orthophoto production workflow.

ProductsA state-wide standard capture program has been created by the LPMA inconjunction with stakeholder and client input. All standard projects have aground sample distance (GSD) of either 50cm or 10cm. A GSD of 50cm is usedfor capturing 1:100,000 map blocks which are based on the NSW TopographicMap Series. These map blocks are approximately 2500km

2in size. Towns with

a population of 400 people or more are captured at 10cm GSD. LPMA hasdeveloped a set of standard products which are produced each time a project isprocessed. The standard products created are dependent upon the groundsample distance of the project.

As the ADS40 is a pushbroom style sensor, the imagery is captured in stripsnot frames. Each strip is processed to two different levels:

1. L1 strips: rectified imagery (triangulated); the backward and nadir lookangles are used to create stereo pairs for a strip

2. L2 strips: orthorectified imagery (triangulated and terrain displacementsremoved); used for creating mosaics

For all standard jobs L2 image strips, orthorectified tiles and mosaics areproduced in true colour (RGB red, green, blue bands) and colour infrared(CIR near infrared, red, green bands). All L1 image strips are produced in truecolour. A product that has been recently developed by the imagery team is themultiband tile and it is available for 50cm GSD imagery. At this point in time,


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there is no demand for multiband tiles from 10cm imagery so they are notproduced unless requested.

Table 1: List of standard LPMA imagery products.

Products (all 8bit)50cm GSD 10cm GSD

CIR RGB RGB

L1 stereo pair strips

L2 image strips

Orthorectified tiles

Orthorectified multiband tiles

Mosaic

True Colour and Colour Infrared Products

The ADS40 sensor captures data in different bands similar to a satellite. Thisfacility enables the combination of different bands to produce different types ofimagery.

Panchromatic band:(465 - 680nm)

Blue band:(428 - 492nm)

Green band:(533 - 587nm)

Red band:(608 - 662nm)

Near Infrared band:(833 - 887nm)

Figure 6: Wavelengths of the ADS40 spectral bands (Leica Geosystems, 2008 andWagner R, 2008).

As part of the standard mapping program, LPMA produce true colour andcolour infrared imagery. A true colour image consists of the red, green and bluebands. A colour infrared image consists of near infrared, red and green bands.

The imagery processing workflow was initially designed to make RGB imagery


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and was then adapted to include CIR imagery. As all the bands are capturedsimultaneously, the same triangulation results, digital elevation model andseamlines are able to be used to create both CIR and RGB tiles and mosaics.

The underlying software used to process the ADS40 strips is called GPro,which is designed by Leica Geosystems. GPro is used to download the imageryfrom the mass memory, run automatic point matching between the imagestrips, produce the L1/L2 strips and perform preliminary colour adjustments. Inorder to produce a clear consistent image, the strips undergo colouradjustments twice in the workflow. Initially, separate histogram break point files(.bpf) are generated for both the RGB and CIR imagery. The .bpfs are createdusing an application in GPro called the Tonal Transfer Curve Editor. They arecreated from 16bit L1 RGB and CIR imagery for a representative strip in theproject. The values for the red and green bands in the RGB .bpf file are appliedto the CIR .bpf, leaving the near infrared band to be adjusted to produce a clear

image, refer to Figure 7. Keeping the red and green band values the same forboth the RGB and CIR strips enables the creation of a multiband tile.

Figure 7: Break Point Files created for the RGB and CIR imagery.


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The .bpf file adjusts the image strip so the colour is as close as possible to theactual on-ground colour, however, it does not fix colour differences betweenadjacent strips. Colour differences between strips can be due to different flyingdates or flying conditions between flight runs. Once the L2 image strips have

been made they undergo further colour adjustments using the colour balancingprogram OrthoVista. All the strips for a project are loaded into the program andare colour matched between strips. This is done separately for the RGB andCIR image strips.

The final colour balanced L2 strips are mosaiced using a program calledMosaicPro which is part of the ERDAS Imagine 2010 suite. All projects areflown with a sidelap of at least 30%, providing the space to create seamlines.The seamlines are manually edited because the automatically generatedseams do not adequately hide the differences between adjacent strips.Differences between strips can be due to temporal changes, lateral

displacement and shadow. All seamlines are feathered by ten pixels to furtherhide the differences.

a) Generated nadir seamlines b) Seamlines after manual editing

Figure 8: Seamline creation (Dubbo Mapsheet 50cm GSD).Imagery projects are available as geotiff tiles for ease of use and file sizemanageability. For a 50cm GSD project, the tile size is 5km x 5km with a filesize of 300MB and for a 10cm GSD project the tile size is 1km x 1km with a filesize of 500MB. The tiles are joined together to produce a final mosaic incompressed ECW format. A final mosaic for a 50cm GSD project is up to3.7GB in file size.


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Multiband Tiles

The multiband tiles consist of four bands: red, green, blue and near infrared.The bands are created to be the same configuration as a satellite image whereBlue = Band_1, Green = Band_2, Red = Band_3 and NIR = Band_4. To view

the tile as a true colour image the bands are assigned red = 3, green = 2, blue= 1. To view the tile as a false colour image the bands are assigned as red = 4,green = 3, blue = 2.

a) True Colour

b) False Colour

Figure 9: Viewing a multiband tile as either true or false colour.


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The multiband tiles are created using the Layer Selection and Stacking tool inERDAS Imagine. The red, green and blue bands from the RGB tile are stackedwith the near infrared band from the CIR tile. Due to the number of tiles in astandard job (around 140), the imagery team developed a batch script for this

tool to automate the creation of these modules.

Figure 10: Layer Selection and Stacking tool in ERDAS Imagine 2010.

As these tiles are created from the RGB and CIR tiles, they have undergonecolour adjustments and are in 8bit format. Traditionally, data used forclassification purposes has been in 16bit format with only limited radiometriccorrections applied. Initial investigations by the imagery team into the use ofthese multiband tiles for low level classification purposes has proven to bepositive, however, users need to be aware that such adjustments can influence

classification results.

Specialised Products

The ADS40 is capable of producing other products outside the LPMA standardproduct range. These include: multiband L2 strips, pan sharpened imagery and16bit imagery.

The L2 multiband strips are similar to the standard multiband tiles and can becreated in both 8bit and 16bit format. Some software packages are unable toread the ADS40 strip files as they are in SOCET SET support file format (.sup),whereas the multiband tiles are a convenient file format and area size.


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Pan sharpened imagery is only created for special cases. Internal research hasshown that it is far better to fly the project at the desired ground samplingdistance as artefacts can appear in pan sharpened imagery, refer to Figure 11.

Figure 11: Artefacts that can occur in pan sharpened imagery.

GPro has the ability to create 16bit L1 image strips, L2 multiband, CIR andRGB image strips. An 8 bit image has 16.8 million possible colour

combinations. Whereas a 16 bit image has 281 trillion colour combinations,resulting in an image that is very rich in information. 16bit imagery istraditionally used for high level classification. There are no colour adjustmentsapplied to 16bit imagery as it changes the value of the pixel and can varyanalysis results. Clients have found the 8 bit products are suitable for theirimage needs including classification.

The file size of 16bit imagery is double that of 8bit imagery causing storageissues for most organisations. For a standard 50cm GSD job one 8bit 4 band(RGBN) strip is 6GB whereas a 16bit 4 band (RGBN) strip is 12GB. Analysing16bit imagery requires specialised software and high power computers which

can also be an issue for some organisations.


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Digital Elevation Models

A Digital Elevation Model (DEM) is created for each project and is used in theorthorectification process. ADS40 DEMs are a by-product of the standardproduction workflow and are not a deliverable product at this point in time. TheDEM for a project is created using enhanced Automatic Terrain Extraction(eATE) software in the Leica Photogrammetry Suite (LPS) created by ERDAS.This software automatically extracts the height of matching points betweenstereo strips. It can take up to four days of computer processing to produce an8m DEM for a 50cm project. These surfaces do not represent the bare earthand contain features such as vegetation and structures. The DEM is refined toa stage where it is suitable for creating orthorectified imagery by manual 3Dediting on Planar screens. The final DEM is stored and can be used again forfuture projects for the same area.

Figure 12: Hillshade of a DEM created using ADS40 stereo pairs (Dorrigo 50cmProject).


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Imagery Geodatabases

The imagery team has a number of spatial geodatabases where datasets thatare collected or created during the workflow process are stored. The datasetsthat are kept are:

1. Flight lines: contains information about each flight line such as time, date,bands, storage, project type and project name.

2. Control points: contains information about the control point such aslocation, source and description.

3. Seamlines: contains information about the seamline such as project nameand image strip it relates too.

4. Tile boundaries: contains information about the tile such as name andarea.

5. Mosaic footprints: contains information about the final mosaic such as

project number, datum, capture date, GSD, accuracy, bands and sensorinformation (basic metadata about the project).

Figure 13: LPMA imagery databases.

The advantage of keeping data such as seamlines, control points and flightlines is that if the area is flown again this data can be re-used and does notneed to be recreated. In the case of seamlines, this dramatically reduces thetime it takes to process the imagery.


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Quality Assurance

To ensure that the products produced for each project are consistent, theimagery team has developed rigorous quality assurance methods. Theseinclude:

1. Standardised naming procedures for all products and intermediateimagery.

2. Standardised folder directory setup to be used for all imagery projects.

3. Colour adjustments are completed on one computer with a calibrated highend EIZO monitor to maintain colour consistency.

4. The horizontal accuracy of the project is checked at the end of thetriangulation and again after the end of L2 image strip creation.

5. Each L2 image strip is checked for image anomalies before mosaiccreation.

To ensure that all operators perform each step the same way a digital manual(DIAS Reference Manual) was created detailing each step of the workflow. Aswell as containing the workflow steps, the manual has a knowledge librarysection detailing any documents or troubleshooting ideas relating to theworkflow. As the imagery team are constantly refining and improving workflowsteps, the manual is regularly updated to reflect any changes so that it alwaysremains current. The manual is also useful for training new staff that join theimagery team (refer to Figure 14).

Figure 14: DIAS Reference Manual.


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The DIAS Job Tracking System was developed in conjunction with the DIASReference Manual. Each workflow step documented in the reference manual isincluded in the job tracking system and has to be marked off when completed.The job tracking database is useful for ensuring that each step in the workflowis carried out, monitoring the status of each job and is used by other sections ofthe LPMA to check when a project is completed (refer to Figure 15).

Figure 15: DIAS Job Tracking Database.


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Future Directions

In order to remain an authority in the production of orthorectified imagery, theImagery Applications team are constantly finding ways to improve theproduction workflow whilst keeping in mind the changing needs of clients. Theultimate aim is being able to produce spatially accurate orthorectified imagery inthe shortest timeframe possible without compromising image quality. As part ofthat process, the imagery team is currently evaluating:

1. Upgrading the Leica Geosystems ADS40 sensor to the Leica GeosystemsADS80 sensor.

2. The latest version of the following software:

- Updated Leica GPro software called XPro, for improved orthorectifiedimage production.

- ERDAS Imagine 2010, for improved orthorectified image and DEM

production.- Leica IPAS Freebird, for improved GNSS-IMU processing.

Conclusion

Since upgrading to the ADS40 digital sensor, the LPMA Imagery ApplicationsTeam has been working hard to develop a set of products that not only promotethe capabilities of the sensor but also meet client needs. When the sensor wasfirst purchased in 2006 very little was known about it or the imagery in Australia.Through client workshops, onsite tours and consultations, LPMA have beenhelping to educate clients on ADS40 imagery and how the products are made.

A huge amount of research has been undertaken by the imagery team withsupport from companies such as Leica Geosystems, ERDAS, governmentdepartments and universities to develop the current product range.

The LPMA standard product range reflects a change in the applications ofairborne imagery. With the introduction of multispectral airborne sensors, usersare no longer restricted to satellite imagery for analysis purposes. Users arenow more imagery aware and expect current, high quality, high resolution andhigh accuracy imagery. LPMA recognises this requirement and has developedthe online NSW Spatial Information Exchange (www.six.lands.nsw.gov.au)which provides access to LPMA imagery and topographic datasets to everyone.

References

Leica Geosystems, 2008, Leica ADS80 Airborne Digital Sensor, DigitalAirborne Imaging Solution. (Heerbrugg: Leica Geosystems AG).

Leica Geosystems, 2007, ADS40 Documentation, Volume 2, TechnicalReference Manual. Version 2.12-86, (Heerbrugg: Leica Geosystems AG).

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

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