Gmap4 = View topos/aerials *offline* and your geolocation

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Gmap4 = View topos/aerials *offline* and your geolocation

Post by Jelf » Thu Sep 25, 2014 1:43 pm

I have posted here before about Gmap4 which is an enhanced Google map viewer that I developed. Up until this summer I believed that Gmap4 would not work if your mobile browser was offline.

I was wrong.

If you follow a few simple instructions, many (but not all) of the Gmap4 features will work just fine on a smartphone, tablet or other mobile device when you are offline. Among other things, when you are offline you can:
* Look at high resolution topo maps (t4 Topo High)
* Look at Google aerials
* Look at other basemaps
* Look at GIS overlays
* Turn on the geolocation feature (Menu ==> My location) and see the map become centered where you are standing
* Move and watch the location symbol follow you and the map automatically move as needed
* Touch the location symbol and see your coordinates in the current coordinate format
* Change the coordinate format
* Turn on/off a UTM grid
* Turn on/off a USNG grid

Yes, you can do all of the above while the browser in your smartphone or tablet is offline. Now let’s pop the hood and see how this works.

Any map you see with Gmap4 consists of a bunch of small image files called “tiles” that are stitched together to cover your screen. When you look at a map while you are online, a copy of each image file is saved in the browser’s cache. If you keep the browser tab open that has the Gmap4 map, then you can then go offline and still pan the map over the area for which there are map tiles in the browser’s cache. If you pan too far, the screen will be white.

To read plain English instructions that I guarantee you will understand (I don’t know how to write computer babble-speak) please visit the Gmap4 Help page and download the pdf file “How To Use Gmap4 Offline”.
Help page:

Gmap4 default map:

Joseph, the Gmap4 guy

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