Check the Satellite Accuracy

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Sometimes the data from the Argos System are not completely accurate, because weather conditions or the bird's posture are not ideal for contact between the satellites and the transmitter on the bird.  The transmitter's battery may be low on power also, and have a weak signal as a result.

The Argos System has a good idea of when these situations happen, and the location data that Argos supplies to us users are classed from highest reliability (class "3") to lowest reliability (class "B"). 

The maps here show where Argos thought the transmitters were at times that we knew that the birds were at their nests on Espaņola Island.  The island is brown and the locations that the satellites reported are red spots.   Remember that the transmitter actually was on the eastern point of the island.   The class B locations were often wrong. 

The satellite system reports a looser cluster of locations for class "B" than for class "A", and a looser cluster for class "A" than for class "0", and so on.  What is clear is that the error increases as the signal quality gets closer to class "B".   Most of the class "B" data are certainly wrong, since they show the birds out on the water at the same time that we saw them sitting at their nests on land!  

However, the amount of error even for class "B" is a few kilometers, compared with the long distances covered on their flights to the Peruvian coast. You might conclude that you would not be very wrong using class B information, and you might want to use class B information if much of the information comes in that class.  You can see below that classes 1, 2, and 3 are pretty good.  In these panels we have magnified the island quite a lot.

Consider doing this kind of check with the Hawaii data; you'll be able to download maps for this purpose from this page in the near future.