Tag Archives: aliasing

More on iPhoto GPS weirdness

Okay, following on from my last post I geek out a lot here, so here’s a summary: In a test of 1600 images, iPhoto moved the recorded GPS location of a picture an average of 6.17m, and in one case moved the image 11.25m from its correct position.

I created a 40×40 array of points approximately 5m (okay, 5 UTM units apart, precisely) and assigned the locations to JPEG files using ExifTool. These files were imported into iPhoto, then exported. The before and after coordinates were plotted and compared:

  • The green crosses are the original coordinates
  • The red crosses are the coordinates assigned by iPhoto
  • The dashed lines map the before coordinates to the after.

In real life, I realise it’s difficult with most consumer GPS units to resolve points 5m apart. It’s pretty egregious of Apple, however, who appear to take great pains to retain all the camera’s metadata, to mash the stored coordinates so badly.

Don’t trust iPhoto’s exported GPS coordinates

John the new jersey geographer put this better than me, but it appears that iPhoto rounds exported GPS coordinates to the nearest integer second of arc. There’s really no reason for them to do this, and it’s caused me to waste several hours tracking down why my tagged and exported photos didn’t match up.

Looking at the output data, I’m not sure if it’s to a second of arc – it appears to be rounding to the nearest hundredth minute, or approximately 0.000167°. Since GPS location uncertainty is in the fifth decimal place, this aliasing of the data is annoying.