View Full Version : Gmap4 = View topos/aerials *offline* and your geolocation

09-26-2014, 15:57
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: http://www.mappingsupport.com/p/gmap4_help.html

From the Help page you can also find the FAQ, examples, a Quick Start guide (also on the Help page) and more.

Gmap4 default map: http://www.mappingsupport.com/p/gmap4.php

Joseph, the Gmap4 guy

Another Kevin
09-26-2014, 16:27
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.

Good to know.

I happen to use Backcountry Navigator, which has similar features for offline download, and integrates some other nice features for orienteering compass, track recording, geotagging photos, and suchlike. (And it lets you create tile sets explicitly : "Please download and cache all tiles from map source A that intersect area B.") One thing that it lacks is a UTM grid overlay (not that I miss it that much, I can ask it for coordinates of a crosshair cursor and read them out in grid form).

Does Gmap4 support NAD27 grids as well as WGS84?

I'm curious whether Gmap4 can integrate with a full custom basemap. If I know that a tile server is at
http://kbk.is-a-geek.net/catskills/tiles/{z}/{x}/{y}.png (and so http://kbk.is-a-geek.net/catskills/tiles/14/4802/5974.png, for instance, is a kind of random tile in the Adirondacks), how hard is it to make the device see that basemap?

And, does that basemap interest you? I invite you to compare and contrast Google Terrain and USGS topos. If you like, there's also a previewer at http://kbk.is-a-geek.net/catskills/test3.html?la=42.1341&lo=-74.1653&z=12, which also shows some of what can be done with photos geotagged at waypoints.

09-26-2014, 17:40

1. Too many new tech map things to work on so I do not see any compelling reason to work on old tech stuff like NAD27

2. And speaking of new tech, your idea of a feature to let the user specify any tile server is great!! There is certainly no technical reason I cannot add that feature. Time to do so on the other hand....
I added your idea to the list. Hopefully I can implement that feature this winter.

Another Kevin
09-26-2014, 18:16
NAD27 is useful only because so many of the US topos predate NAD84 and are marked using it. Also, a number of state coordinate planes are NAD27-based. Sure, it's old, but there are a lot of maps in the wild where that's the grid.