15arspc submission 90

8/8/2019 15arspc Submission 90

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Figure 2 Progressive zoom on typical GPS-surveyed point

Completed survey packages included the surveyed location of GCPs, sketchesof the feature chosen, digital photographs of the area and the point identified inthe reference ALOS PRISM imagery.

Accuracy assessmentTwo sets of ALOS PRISM nadir images were used for preliminary accuracyassessment of the orthorectified imagery derived from the strip adjustmentprocess.

One set of data formed an approximately 2000 km long, 35 km wide strip of 80images from the same orbit over Eastern Australia, as shown in Figure 3. The

second data set formed an approximately 730 km long, 35 km wide strip of 26images located from Lower Darling Basin to Victoria coastline, also shown inFigure 3.


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Figure 3 Locations of ALOS PRISM nadir scene strips

For the 80-image long pass, there are around 200 ground points availablecollected from a variety of sources, the majority of points were GPS-surveyed to1 m or better accuracy in both planimetry and height. Ten GCPs collectedthrough the NGCP project were used for strip adjustment - four at the northernend, two in the middle (near Canberra) and four at the southern end. The rest ofthe points were used as check points. The distribution of GCPs and checkpoints is shown in Figure 4. The RMS error of checkpoint discrepancies isaround 1 pixel (2.5 m) across track and around 1.4 pixel (3.5 m) in the alongtrack direction. Orthorectified images were then generated from the strip forassessment.

A continuous series of points, defining the three dimensional location of roadcentrelines, are available for checking against the orthorectified imagery. Thesepoints are acquired through differential Global Positioning System (dGPS)technique and they have a planimetric accuracy of 2 m. The distributions of thedGPS tracking data are also shown in Figure 4.


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Figure 4 Distribution of GCPs, check points and dGPS tracking data

Visual examinations show that dGPS tracking data and the orthorectifiedPRISM imagery are in good agreement to pixel (2.5 m) level. Figure 5 illustratesexamples of visual assessment.


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Figure 5 Example of dGPS tracking overlay on the orthorectified PRISM imagery

For the 26-image shorter pass, there are 18 ground points available all of whichwere collected through the NGCP project. Points surveyed from the NGCPproject have an accuracy specification of 0.25 m in planimetry and 0.5 m inheight. Eight GCPs were used for strip adjustment - four at the middle and fourat the southern end note that no ground control was used at the northern endof the pass. The rest of the points are used as check points. The distribution ofGCPs and check points is shown in Figure 6. The RMS of checkpointdiscrepancies is around 0.6 pixel (1.5 m) across track and around 0.7 pixel (1.8m) in the along track direction. Orthorectified images were then generated fromthe strip for assessment.

High accuracy Light Detection and Ranging (LIDAR) digital surface models(DSMs) were available for checking against the orthorectified imagery. Thevertical accuracy for this dataset is 0.15 m and the planimetric accuracy is0.25 m. The DSM is in 1 m resolution and road intersections were shownclearly on this DSM. The location of the LIDAR data is also shown in Figure 6.


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Figure 7 Example of LIDAR data (left) and orthorectified PRISM imagery (right)

Reference mosaicFollowing collection of field data, packages were examined for completenessand accuracy and then ingested into a database. At present, these primaryGCPs are being utilised to georeference passes of ALOS PRISM imageryutilising BARISTA. The passes thus orthocorrected will be used to create anational mosaic reference image.

Secondary control pointsWhilst BARISTA has demonstrated great utility in the accurate orthocorrectionof long passes of satellite imagery using only a few control points at each end of

the pass, there are occasions when orthocorrection of a single scene is desired.In order to accommodate this, each scene in the reference mosaic is beingexamined for suitable features for use as secondary control points (SCPs).

Some twenty or more SCPs are chosen in each scene in the reference mosaic.These points are also being stored, along with an image chip, in a database sothat a continental-wide coverage of control points at high accuracy is obtained.

To evaluate the accuracy of the orthorectified imagery generated from SCPs,the 80-image long strip over Eastern Australia was used for testing. SCPs weregenerated from three orthorectified images as shown in Figure 8. The heights ofthese SCPs were extracted from Shuttle Radar Topography Mission (SRTM)

1 second (~30 m) data. SCPs were then used for strip adjustment to derive thenew orthorectified imagery over the same pass. The strip adjustment resultsbetween NGCP GCPs and SCPs are almost identical the differences for thecheck point residuals are between 0 m and 0.2 m (~0.1 pixel).


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Figure 8 Orthorectified scenes used for SCPs generation

An intensity-based image matching technique (cross-correlation) was used todetermine the sub-pixel differences between the orthorectified imagery derivedfrom NGCP GCPs and SCPs. Some 625 (25 by 25) gridded points wererandomly selected as candidate matching points; of these, 623 points weresuccessfully matched with correlation coefficients greater than 0.7. The meandifference of these matched points is smaller than 0.1 pixel and the difference instandard deviation is smaller than 0.3 pixel. The result shows that orthorectifiedimagery generated from SCPs agrees well with orthorectified imagerygenerated from NGCP GCPs.

Both the reference mosaic and the control point database will be released undera Creative Commons Attribution licence to encourage their widespread usewithin the spatial community.


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ConclusionPixel level accuracy has been demonstrated for orthocorrected ALOS PRISMscenes using relatively few GCPs at each end of a satellite pass. Highaccuracy has also been demonstrated for a significant number of scenesoutside the controlled area permitting accurate correction of peri-coastalfeatures.

The use of secondary control points derived from the national reference mosaicimagery has been demonstrated to yield orthorectification resultscommensurate in accuracy with surveyed control points.

The pending release by Geoscience Australia of the national reference imageand the control point datasets under Creative Commons licensing will providethe spatial industry with a consistent and accurate reference.

Acknowledgement

The permission of the Chief Executive Officer of Geoscience Australia topublish this paper is gratefully acknowledged.

ReferencesFraser, C. S., Ravanbakhsh, M. and Awrangjeb, M., 2009, Precisegeoreferencing in the absence of ground control: a strip adjustment approach.International Archives of Photogrammetry, Remote Sensing & Spatial Information Sciences , Hannover, Germany, Vol. 38, Part I-4-7/W5.

Rottensteiner, F., Weser, T., Lewis, A. and Fraser, C.S., 2009, A StripAdjustment Approach for Precise Georeferencing of ALOS Imagery. IEEE

Transactions on Geoscience and Remote Sensing , 47 (12; Part I): 4083-4091Rottensteiner, F., Weser, T. and Fraser, C.S., 2008, Georeferencing andorthoimage generation from long strips of ALOS imagery. Proceedings of 2 nd ALOS PI Symposium , ESA/JAXA, Rhodes, Greece, 3-7 Nov., 8 pages.

15arspc submission 90

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