Hi Katherine --
With Collector, there are two ways to get a basemap offline:
- Make a map package (TPK) that you'd sideload to the device. This isn't supported with the World Imagery basemap available through ArcGIS Online.
- Download map areas through the Collector app. This can use the World Imagery basemap available through ArcGIS Online, but does have limited size for each map area. I did a test using the World Imagery basemap in my area, and I could make a 16,000 acre area at 1:5000 scale (you'd need to try this for your areas of interest, as the size of the packages isn't an exact mapping to the size of the area and the scale -- I found my map area to be about 9MB).
For downloading map areas through Collector, you have two options, too. One is that the app user defines the area to take offline (creating on-demand map areas), and the other is to make preplanned map areas in ArcGIS Online and have your users just download them in the app. It sounds like preplanned map areas are going to be a good choice for you: In ArcGIS Online you'll specify areas and their levels of detail, and then in Collector your mobile workers pick the areas they need and download them. (Since you are using iOS, I've assumed you are using the latest Collector app, as Collector Classic doesn't support preplanned map areas.)
You can learn more about creating preplanned offline map areas in Create, edit, and manage map areas in the ArcGIS Online help. You can learn more about making maps for offline use in Collector in Prepare for offline data collection in the Collector help.
Esri Education Team