I researched GPS units and most had some type of problem/issue. I got an app for my iphone called gps4cam. To use this app, the iphone saves the geotag data during your trip/photoshoot and operates independently of the camera. At the end of the photoshoot you export the data by emailing it to yourself. You must also take a jpeg of a QR code on the iphone (multiple QR codes if a long trip) which you will upload to your desktop at home.When you get home you use an associated desktop app to geotag your photos on your computer.

I just returned from a 1 week workshop in Italy and this app worked great.
You can pause the trip and resume at any time. The data is not lost if the iphone runs out of battery or is turned off.

Hi Richard,
I use this method to get GPS info onto my photographs:
Purchase an Eye-Fi SD card an insert into the camera.
Install the Eye Fi software onto a smart phone or tablet.
In "settings" select "use device location to add location to my photos"
Take a set of photos and they will be sent to the smartphone or tablet.
When they are loaded onto your smartphone or tablet the photos will be "Geo Tagged" using the devices location.
I use an LG G3 tablet which fit neatly into my camera bag, or my LG G2 phone when not using my camera bag.
Hope this helps,
John.

Hi Richard,
I use this method to get GPS info onto my photographs:
Purchase an Eye-Fi SD card an insert into the camera.
Install the Eye Fi software onto a smart phone or tablet.
In "settings" select "use device location to add location to my photos"
Take a set of photos and they will be sent to the smartphone or tablet.
When they are loaded onto your smartphone or tablet the photos will be "Geo Tagged" using the devices location.
I use an LG G3 tablet which fit neatly into my camera bag, or my LG G2 phone when not using my camera bag.
Hope this helps,
John.