I don't know of any purely offline tool, but personaly I use obsmapp (tool for observado.org). You can record sightings offline, based on gps-location and Open Street Maps. When you have an internet connection you then upload the whole lot to the website. You can choose 'obscure site location' standard which prevents localities being shown to all other users except admins and yourself. I usually obscure most observations I record but do upload them so they are in the database and can be used for distribution research if needed. Of course you can also export your own sightings to .csv and then remove all of them again from your account (though not really the use of the free app of course
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