I was reading through the Taxacom mailing list over the weekend and saw a few posts about GPS accuracies and what you need to record from your GPS.  I was completely aghast at one comment…

“The datum when recorded doesn’t matter…”

It’s taken well out of context but the statement in itself caused a few people to write in that the datum always matters, and I can’t help but agree.  So, if you’re recording with your GPS, what’s the best practice?

• Always record the X and Y values (obviously!) – and to an appropriate number of decimal places!
• Always record the projection and datum (usually just datum, because you’ve set your projection earlier)

That’s the core of what I record when I’m doing work, but being a bit OCD, it’s also wise to think about:

• Accuracy (if your unit gives you that in meters/feet, or if it just gives you a DOP value, record those)

If you’re doing altitudinal work, then the horizontal datum is also something to consider.

How does this work for me?  Well, it’s been a while since I’ve managed to get out in the field – even to do my hobby bird surveys – but usually I record the datum and the X/y (sometimes even in two projections, like below).  Originally I used a lot of data in Australian Map Grid based projections, but then as I started to do surveys around other parts of the world, good ol’ geographic co-ordinates started to be more of a friend to me.

(If you haven’t seen my digital bird database, check it out here, but promise not to laugh…)

Interestingly, some of the new GPS enabled cameras (like the Caplio 500SE, review here, and yes, we re-sell it) store this information in the image files that they download (e.g. in EXIF tags).  Or you can use other systems like the Sony Image Tracker and others to post-match your images.  There’s heaps of ways to tag images out there, anyway.

But the core of this blog post was aiming to say:

Always record the datum!

Sometimes even twice…

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