Saturday, May 22, 2010

Aerial photography and shoreline mapping : checking accuracy

Example of difference between aerial imagery and chart
(Ngmararu point in New Zealand)

Aerial photographs are the primary source material used to create coastal survey maps.
These data sets, in turn, provide information to Hydrographic Services for producing official nautical charts.

Combining information from aerial photographs with hydrographic data helps to ensure that nautical charts are accurate.
Nautical charts are one of the most fundamental tools available to mariners for planning voyages and navigating ships using the shortest, safest, and most economical routes.
So it is obviously important that the information displayed on charts is correct.

Notes about accuracy :

  • regarding nautical charts :
When international Hydrographic Services conduct hydrographic surveys, they used the best technology through stringent accuracy standards for the collection of data by their survey vessels. But of course only for the moment in time.
Coastal waters are extremely dynamic. Natural shoaling occurs, earthquakes move sea beds, channels are dredged and new wrecks and obstructions are discovered.

So the necessity for our web service to propose regular chart updates (which should match at term to the frequency of updates proposed by the different Hydrographic Services) and for the user to take into account the information promulgated through Local Notice to Mariners.
Don't forget that for surveys performed prior to the mid 1990’s (so before standard DGPS), the accuracy requirement was only 1.5mm at the scale of the survey. On a 1:20,000-scale harbor approach survey, an accuracy of 1.5mm on the chart equates to 30 meters in real life.

RNCs are made by scanning the paper chart printing materials.
Any inaccuracies due to old methods of collecting, processing and displaying data on the paper chart were transferred to the RNCs. As a result, the accuracy of modern positioning systems such as GPS may exceed the positional accuracy of the RNC. The impact of positioning accuracies can be minimized by not zooming an RNC beyond the scale of the original chart.
For example, while NOAA has accuracy standards for each step in the data collection and chart production process, much of the depth information found on official nautical charts is based on surveys conducted before 1940, the shoreline is more than 20 years old, and paper charts used to be compiled manually.

By the way, this also means that the prudent mariner must use the
Route planning tool on the Marine GeoGarage viewer to set waypoints to pass shoals or isolated dangers with utmost caution, no matter what navigation method is used. For navigation purposes, it's always necessary to keep a safety margin...

  • regarding Google Maps imagery :
The vertical aerial photographs used in Google Maps have been georeferenced to align with the coordinate system.
The process of georeferencing involves identifying ground control points in the image for which accurate coordinates are available. A transformation is then calculated by computer software which processes the image so that it aligns to the ground coordinate system (Wolf and Dewitt, 2000).
Mosaics are used to stitch many aerial photographs together.

Controlled mosaics use rectified photos so that all of the photos are vertical and at the same scale. In mosaic assembly, image positions of common features in adjacent photos are matched as closely as possible. A plot of control points is used to match and constrain positions, similar to the technique used in georeferencing.
Uncontrolled mosaics simply match the image details of adjacent photos without using the ground control, which is quicker but less accurate in terms of the coordinate reference system.
Semicontrolled mosaics have either no ground control or use photos that have not been rectified.

So Google Maps are certainly not to be considered a "gold standard" for accuracy as they provide a seamless worldwide imagery display. Google buys or licenses the imagery from different sources using different quality standards and referred to different datums, to stitch it all together to make up their product in an unique WGS84 datum.

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