For most surveys, you do not need to worry about Offline Maps. When on-site, if you have a good/fair connection to the internet then the OTISS survey apps will be able to display the required maps and use them for your surveying. The survey apps will automatically store parts of the maps on the phone/tablet to increase performance and reduce 4G/5G data usage – but you don’t have to do anything.

However, for areas of poor 4G/5G mobile signal, you will want to pre-load the maps before you leave your home/office. So you need to read this page and use the Download Maps feature on the OTISS survey apps.

What are Offline Maps?

OTISS provides you access to the worldwide OpenStreetMap and Google road/aerial maps, and to the UK Ordnance Survey maps (not OS MasterMap). And, on the OTISS website, you can also upload your own maps, drawings and plans and assign them to the survey sites. The mobile survey app lets you view and zoom around these maps – downloading the appropriate map tiles from the internet as they are needed, i.e. from the ‘online’ OpenStreetMap and OTISS map servers.

The survey apps can also download the relevant parts of these maps to the phone/tablet. These map tiles/images are now stored ‘offline’.  These ‘offline’ maps are then used rather than reloading the ‘online’ images from the internet.

What are map tiles?

Map tiles are square bitmap graphics that are displayed in a grid arrangement to show a map. OTISS, and the other internet map providers, break up large maps into a grid of standard sized map tiles. This app works in the same way as your internet browser – it only downloads those tiles that are needed to display the visible area at the current zoom level. Panning or zooming the map will require a new set of tiles to be downloaded.

The UK Ordrnance Survey use ‘Vector Tiles’. These are not images, but collections of features/vectors – and they can cover a larger area. When downloading the OS maps, we can download a lot fewer of these vector tiles and cover a larger area.

Auto Offline Maps

When the Auto Offline Maps feature is enabled, as you pan and zoom around a map, the map tiles are automatically stored ‘offline’ on your phone/tablet. This is the normal mode of operation – you don’t need to do anything. Press the Settings Icon to turn the Auto Offline Maps on and off.

Download complete map of UK & Ireland

Premium users can download and use a complete, offline OpenStreetMap of ‘UK & Ireland’. Of course OpenStreetMap only provides a certain level of detail and is not suitable for planning surveys – but it is very good and can be used for most tree safety and condition surveys.

Select any survey and goto the Download Maps page. Use the Layers icon to select ‘UK & Ireland’ and you will see a blank map. Now press the Download button and follow the instructions provided. See the device’s Notification bar for progress of the download. It may take a 20 minutes or so, and will use up 2GBytes of storage, but once downloaded, you can use this map anytime – regardless of mobile signal strength.

NB: There is quite a bit of processing and unpacking when using this offline maps, therefore on some phones/tablets it may take a few moments looking at a white screen before the maps appear.

Download Shapefiles

When Shapefiles were loaded onto the OTISS website, you ticked a box to say whether they had a large (i.e. > 5000) or a small (5000 or less) number if items.

  • When you open the survey in Start Survey or Download Maps, the small shapefiles are automatically downloaded in one go and immediately stored for offline use. The shapes are then drawn as vectors – usually overlaid on one of the base maps.
  • The large shapefiles are treated as a large, tiled, raster image and downloaded as explained below for the maps and site plans.

This definition of large/small is a bit vague and it just a matter of performance on your PC and phone/tablet.

Download UK OS maps

The OS maps (not OS MasterMap) are downloaded as described below, but the UK Ordrnance Survey use ‘Vector Tiles’. These are not images, but collections of features/vectors – and they can cover a larger area. When downloading the OS maps, we can download a lot fewer tiles and cover a larger area.

NB: There is quite a bit of processing when using the UK OS maps, therefore on some phones/tablets it may take a few moments looking at a white screen before the maps appear.

Download Offline Maps

For survey areas with poor 4G/5G signal, you can preload the offline maps before leaving your home or office (i.e. with a Wi-Fi connection).

  1. Download a survey to the mobile device.
  2. From the list of Downloaded Surveys, you can select a survey and goto the Download Maps page.
  3. The Layers menu can be used to select the map layer you wish to download – typically your site map/plan, Google or OpenStreetMap.
  4. Pan and zoom to the appropriate section of the map.
  5. Everything you see on the map will be downloaded at all zoom levels.
  6. Press the Download button and wait for the messages to say that the download is complete.
  7. The progress is reported below the map. The OTISS map server will collate all the tiles required and download them in a single large file. Whereas for Google, OpenStreetMap and UK OS, we download the map tiles individually.


  • Larger sections take longer to download – this is because a separate set of map tiles must be downloaded for each zoom level. So it will be quicker if you select only the area you actually need. The Download button will disappear if the area is too large to download in one go – just zoom in a level or two.
  • Large maps will need to be downloaded in several sections.
  • For larger sites, the lowest zoom level(s) of the map may not be needed or may not be clear enough. In this case, you can reduce the “Max zoom level” (e.g. from 24 to 22 or 21) to remove these lowest levels. This allows for faster downloads of the same area, or being able to download larger areas – i.e. without the lowest zoom levels.
  • For Google, OpenStreetMap and UK OS; repeating the Download may be useful: (a) to verify that all the map tiles have been downloaded; or (b) to retry the download if the previous attempt failed. In all cases, only new map tiles will be downloaded, the existing ones will be checked (but not downloaded again).

Offline Storage

These offline map tiles do take up storage space on your phone/tablet. It is recommended that you delete the offline map tiles for a survey when they are no longer needed. These tiles are automatically deleted when you remove the survey from the app.

Redraw Map Icon

Occasionally, the map displays blank or pink squares when it cannot immediately find the correct map tiles to display – whether from ‘online’ maps. This may only be a temporary issue, or may be because you have lost contact with the map servers. In any case, pressing the Redraw Map icon will redraw the map again, and hopefully fill in the missing map tiles.


Turn the automatic offline caching on and off.

Press the Delete button to Delete button to remove the offline map tiles for this survey.