Mobile GIS & LBS
Matt holds an MSc in Geography and GIS. He has been working with clients solving problems with GIS for over 17 years. Matt founded WebMapSolutions whose mission is to put innovative, intuitive GIS driven applications into the hands of new and existing users.
Basics: Offline Base-Maps and Layers – Part 2
August 30th, 2016 by Matt Sheehan
In our first article in this series: Basics: Offline Maps and How to Get Started Part 1, we provided background on offline maps. We discussed the difference between base-maps and layers, and covered what are interactive and editable map layers. In this second post in the series we will discuss how we take base-maps and layers offline.
1. Taking Base-Maps Offline
For this conversation we will focus much of our attention on ArcGIS. Note, the same basic principles discussed here will apply to other map and GIS offerings. So what is a base-map? It is a series of pictures or tiles combined into a single image. Rather like a jigsaw puzzle. And just like a jigsaw puzzle a single image is usually broken into smaller pieces. Think of a huge satellite image. Take a cookie cutter and break that up into smaller chunks. These are tiles.
So why generate tiles? Improved user experience that is why. Google in 2005 released Google Maps. Suddenly we could pan and zoom satellite images for free on the web. And yes the first thing I did was zoom in to see my house. But take a closer look at Google Maps or an ArcGIS web map today. You will see the individual base-map tiles as you pan and zoom. Your experience is seamless and super fast. Imagine each time you panned the map, just for a moment the map disappeared, then reappeared with the new extent. That was the pre Google Maps reality.
So how does this relate to offline? To be able to use base-maps in areas without wireless connectivity we need to copy these tiles to our smartphone or tablet. In Esri-land this can be done dynamically (see Collector for ArcGIS and the base-map selection option for offline) or through a tile package or TPK. This is simply a zip file of images. Tiles at different zoom levels: a “pyramid” of images.
Why tiled maps? from Penn State is a good article on this subject, well worth reading.
So we physically need to copy a “pyramid” of images to our mobile device. Two questions may come to mind here:
1. How long does it take to download these images?
Let’s play devil’s advocate. You would like to download satellite imagery as a base-map for the State of Utah. You would also like to be able to zoom to a level to be able to see buildings. That is many tiles, all large in size (remember we are dealing with png’s or jpg’s). We could be talking here in terms of 20 Gigs of data or more!
We will revisit this base-maps and capacity conversation in our next post
2. Taking Map Layers Offline
There are many map layer formats. But only one is use-able in offline mode. In Esri-land that is called a Feature Layer. So what is so special about a Feature Layer? It is not a series of pictures (points, lines, polygons) overlaid like an acetate over the base map. A Feature Layer is the raw data of a layer. For those familiar with this term; think of a JSON file. You could actually read the contents of a Feature Layer. In the web world, your browser reads this data and renders (draws) this data onto the map. That gives us some very clever capabilities. It means that each feature on the map (road, lake, power pole) is interactive. We can tap a feature with our finger and up pops an information box with information about that feature (height, width, date last inspected etc). We can also make edits: add, update and delete a feature. Feature Layers are super useful. The image below shows our AppSmart for ArcGIS web app fully disconnected. A local base-map, and Feature Layer: points, lines and polygons are shown.
So again what does this have to do with offline? If a Feature Layer is data rendered by the browser, we simply need to copy this data to our smartphone or tablet and hey-presto we can work with these layers in a completely disconnected mode.
That’s it for part 2 in this series. We will begin to pull the pieces together in our next post and discuss the different ways maps and GIS functionality can be used offline. Until then …
Feel free to contact us on 801-733-0723 if you have questions.