• Caching

  The easy (but limited) way. If these three layer options are sufficient for your mapping needs and the field area is not overly large, you can temporarily "cache" the imagery to the iPad's internal memory. Think of the cache as a rolling temporary memory- once the cache limit is reached the most recently viewed imagery overwrites the older imagery. Google Earth and most web browsers operate similarly. To cache imagery you should zoom on the map to the maximum imagery quality that you desire. Once the imagery for this area is loaded (clearly visible), move to the adjacent area and let that load. Repeat this until you have covered the entire field area. Repeat this procedure with the other imagery types for the same area if desired. Then double-check that when you turn off the WIFI the imagery is still visible (this indicates it is temporarily stored, and will remain stored until you clear the cache or overwrite it by loading other areas after). Note in FieldMove's Settings menu there are relevant options for basemap loading and cache resetting. I usually make sure caching imagery is the last thing I do before heading to the field and then leave my iPad on Airplane Mode.

  This method works great as a "quick-and-dirty" option, requiring little time or setup!

• Importing

  The harder (but more empowering) way. Arguably one of the most important features of FieldMove is the ability to import your own basemaps. For the iOS case described here this means using iTunes as the bridge between your computer and iPad. Note that FieldMove does not currently support vector-based file types for import. Two raster-based file types are supported for importing, GeoTIFF and MBTiles.

  • GeoTIFF: GeoTIFF is a fairly standard file format which can be read by most GIS software packages including ArcGIS and Google Earth Pro. It is essentially a TIFF image file with georeferencing information embedded in it (projection, coordinate system, extent, etc.). These files are typically easy to generate (e.g. in ArcGIS). A key disadvantage of this file format is the often large file size for map areas and the fact that FieldMove will try to load the entire file at once (not just the portion visible in the current map view). This can lead to slow basemaps load times, especially initially. I would not recommend importing GeoTIFF files larger than about 350MB each.
     
  • MBTiles: MBTiles is the preferred file format, though in some cases may require an additional step to prepare (such as converting from a GeoTIFF file). As the name suggests, this single file is actually a database of smaller raster images that are referenced to each other according to a tiled mosaic. This file structure allows you to customize what is visible at different map scales to ensure the best balance of resolution vs. file size, detail vs. clarity. Many web-based map applications use MBTiles for its efficiency such as Google Maps (with slow internet connections you will see the map area load square by square). The advantage of this file format is that only the imagery within the current map view is loaded (and at the appropriate scale), making for much more efficient basemap loading in FieldMove.

  MBTiles work off the principal of a map zoom level, which is defined by the width a 256 pixel wide map is in degrees longitude. A tile is a square raster image 256 pixels by 256 pixels. Notice the steep climb of the number of tiles as zoom level grows (and file size correspondingly). Although this means the pixels per meter depend on latitude and the map scale depends on the pixel size/density of the viewing screen. Some generalizations based on maps centered on the equator are provided in the table below. CalTopo supports exporting images to zoom level 16. FieldMove and Move software support basemaps to zoom level 20. It will depend on the type of mapping and applications you require, but generally zoom levels 16-18 work well.

Georeferenced Map Sources

• CalTopo

  CalTopo is an invaluable (free, account recommended) online GIS, which compiles many excellent basemaps and has functionality to efficiently export to the MBTiles format. In my opinion CalTopo is one of the more user-friendly, and underappreciated resources on this list (hence my longer explanation). Although beyond the basemap discussion here, CalTopo allows you to build your own maps online, import/export/create point, line, and polygon data, collaboratively edit maps, and offers many analytical features (measuring tool, create a viewshed, create a profile view, calculate sun exposure, and calculate terrain statistics for a selected area). Really quite incredible features for a free homegrown piece of work and I highly recommend exploring CalTopo's many capabilities! For frequent users I recommend a Pro account, highly worthwhile at $50/yr. Among other features you can export much larger areas as MBTiles.

  Coverage types include shaded USGS topographic maps, satellite orthophotos, shaded relief (hillshade), road maps, historic maps, and terrain maps, and include the ability to overlay contours, slope shading, land management data, fire history data, as well as current weather, water, and snow data. Coverage areas are typically complete across the United States, with some data (topo, etc.) available for Canada and New Zealand. Google layers will work globally.

  Due to licensing restrictions it is not possible to export Google or SNOTEL layers, but all others can readily be exported to MBTiles format. This process is walked through below:

  1. Setup. Navigate and zoom to desired field area. Select the desired layer or combination of layers from the left column or drop down menu in the top right corner (mouse over text there, click on text right of "Base Layer" to select, add additional layers as desired, select checkboxes below as needed.

  2. Export. Once you have the desired basemap in view (+/- overlaid data), mouseover "Print" from the top toolbar and then click on "Download KMZ or MBTiles File". This opens a new window or tab in your browser which should have a red rectangle with arrows in the corner (export area). Click-and-drag an arrow on the corner of the rectangle to adjust the area to be exported. Notice because of the tiled nature of the data you may have to download a slightly larger area than targeted for full coverage. On the left size is an option to select the desired target zoom. The highest export option is likely to be "16 1:12k", which is what I would select unless I am worried about file size. When ready, click on "Download MBTiles" to have Caltopo prepare to download the file to your computer. Once downloaded I would recommend immediately renaming the file something meaningful so you can distinguish it from other files you create (e.g. Hill40ftContUCRcampus.mbtiles; you cannot rename them once they are sent to the iPad).
     
     * Hint: If you want to make the map area clear to students and avoid them walking out of it, add a polygon defining the map area before exporting.

  3. Import. Use directions below to import the created MBTiles file(s) into the FieldMove directory via iTunes.

  Site:http://www.caltopo.com

  How-To: http://help.caltopo.com/kb

• OpenTopography

  OpenTopography (free, account recommended) specializes in access to high resolution topographic data, especially publicly-funded airborne lidar data. In addition to raw point cloud data download (for advanced users), there is functionality to have OpenTopography process KML (Google Earth) and GeoTIFF files for you. Current high resolution coverage is mostly limited to specific areas in the United States, but global 90m Shuttle Radar Topography Mission (SRTM) data is also available. The site also features excellent tools and tutorials to support topographic analysis.

  Data: http://opentopo.sdsc.edu/datasets

  How-To: http://www.opentopography.org/learn

• Koordinates

  Koordinates (free, account required) is a useful clearinghouse for geospatial data. New Zealand, United States, Canada, and Australia are best represented but new data seems to be added all the time. Vector datasets will need to be converted to raster using a different program.

  Data: https://koordinates.com/data/

  How-To: https://help.koordinates.com/using-data/

• EarthExplorer

  EarthExplorer (free, account required) is USGS's main search and download system for orthophotos, elevation data, land cover data, and diverse satellite imagery. If you know exactly what it is you are looking for (or have the time to browse), this is a good way to get it. In most cases it will be left up to you to convert the data to a GeoTIFF or MBTile.

  Website:http://earthexplorer.usgs.gov/

  How-To:  https://lta.cr.usgs.gov/ee_tutorial_search_criteria

• The National Map

  The National Map (free, account required) is USGS's outlet for topographic data (specifically as USGS topographic maps and elevation data). In most cases you will need to convert the data after download.

  Website: https://viewer.nationalmap.gov/launch/

  How-To:http://training.usgs.gov/TEL/TheNationalMap/TNM-TEL-Index.html

• TopoView

  TopoView (free, no account required) is yet another USGS outlet for topographic maps. I have found their server to be rather slow at times, but otherwise it has an excellent interface for searching an area for published USGS topographic maps of different ages. It offers the option to download all maps in a GeoTIFF file format. Since the file sizes are relatively small (10-20MB) you do not really even need to convert them to MBTiles. Easy!

  Website: http://ngmdb.usgs.gov/maps/topoview/viewer/#4/40.00/-107.51

  How-To: https://www.youtube.com/watch?v=kOpe3WXsZrQ