Comparison - Google Earth Georectification vs Landsat ETM+ Georectification. Who is Right?
My previous post elicited a question asking which image was georectified properly and wondering if I had verified the projection. I am a trusting soul and if, in the instance of my previous post, the NGA PGS vector coordinates are what I used to georectify the ISS image. I am reasonably confident the ISS image fit precisely into the NGA PGS vectors. Ultimately the ISS image I fashioned used a Geographic (latitude/longitude) projection with WGS-84 datum.
This evening I loaded the Landsat ETM+ orthorectified mosaic N-20-10_2000, which includes Isla Blanquilla, into Global Mapper. It loaded quickly. I zoomed into Isla Blanquilla at a scale of 1:62,5000, which was the scale of my final map project. I then loaded the ISS georectified base image of Isla Blanquilla on top of the Landsat image. It is right on! A precise fit. However, yesterday when I attempted to create an overlay of the ISS image in Google Earth and after spending 15-minutes stretching and twisting the image, it never quite fit. It was alway alittle off.
Google Earth to Landsat ETM+ Georectification Offset
This evening, I recorded the lat/lon location of three relative ground control points within Google Earth and in Global Mapper's Landsat display. They each showed the same image of Isla Blanquilla and supposedly at the same location. Each relative ground control point was easily discernible. After recording lat/lons for each point and determining the difference, it seems someone is off anywhere between 2-meters and up to 63-meters for this single island. So, who is it? Who has misplotted this island? Is it Google Earth or Landsat ETM+? Care to guess?
Enjoy!
Labels: georectify, Google Earth, ISS Image, Landsat ETM+
I wouldn't trust Google imagery's positional accuracy for one second! I've found dozens of locations where their georectification doesn't match any number of other sources (all of which agree with each other.) It's truly horrible in some places where their imagery is off by hundreds of feet.
It is quite scary knowing that a whole body of data is being developed (the KML community, etc) with such horrible base referencing. Even more scary is that so many people don't even notice.
Posted by
perrygeo |
12/01/2007 11:25:00 AM
Matt brings up some good points about GE's imagery etc. I would say that all parties involved are adding to the positional error.
Think about the distances involved: 60 meters (!)? At this scale 60 meters is not much: 2 pixels in a LandSat image, or 4 pixels in a LandSat panchromatic image. It is very possible that the positional error associated with original products, any conversion between coordinate systems and any subsequent rectification would be on the order of 100+ meters.
Dylan
Posted by
Dylan |
12/01/2007 03:05:00 PM
A Japanese manufacturer was tasked by a US firm with a job of producing 100,000 widgets. The job order allowed for 1 problem widget for every 1,000 produced. The Japanese firm was surprised by the job order specs, but produced the widgets and delivered them to the US firm. There were two bags of widgets. The US representative asked what these two bags contained. The first bag contained 100,000 widgets, less the 1 problem widget. The other bag contained the 1,000 problem widgets.
Google Earth needs to produce 1,000 georectified error-free plots for every 1,000 they plot. Leave the errors to the other guys.
Posted by
Mr Minton |
12/01/2007 04:26:00 PM
Both the Landsat imagery and DigitalGlobe imagery have been orthorectified to a 1:50,000 National Map Accuracy Standard. The DigitalGlobe imagery is processed without using ground control and with a basic elevation surface. With proper ground control and elevation, an image from DigitalGlobe can have submeter accuracy whereas a Landsat image can not.
Posted by
geoguy |
12/14/2007 07:41:00 PM
The comparison Georectification vs Landsat ETM + Georectification is quite interesting.Anyway thanks for sharing.
regards
Orthorectification
Posted by
Maps digitization |
4/23/2009 03:26:00 AM