Way to go getting the Nano Logger logging! The first elevation data point is always wacky because the first barometric pressure data point is always wacky. Elevation is computed by the sketch directly from pressure (hPa). Note that the first temperature data point is also erroneous and always the same.
I don't know whether these first data points are default values inserted by the library when no real data are available or whether they are more random. I do know that they should be ignored.
Don't forget to edit the sketch so that the current sea level barometric pressure is entered before you use the logger to save real elevation data. You can do the conversion later from the hPa data if you know the formula, but you have to know what the barometric pressure was when the data were collected. Get the sketch here.