Android application for viewing GPS position of some users on a Map

INTRODUCTION I'm developping an android application in Android Studio, and I want to add some functionalities regarding AVL (Automatic Vehicle Localization - maps, mobile localization, etc...). My application will send periodically the mobile GPS coordinates to the server ad will receive from the server periodically other mobile users (Android) GPS position, so I want to view them in a map, on Android . TARGET FOR THE DEVELOPPER 1) To guide me in selection of maps to be used (for example Google Map APIs, Open Street Map , etc... ), discussing with me about the choose. The Android mobile could not be ever connected to public internet, then it has to hold the maps (at least part of the map regarding the interested area) locally (offline usage) . It is always connected to a private network to the server, but sometimes cannot be connected to public internet, so it needs to cache locally the maps. 2) To suggest me a system to reduce the used resources (bytes sent for GPS position, CPU usage) in order to send the GPS position from the mobile users to server (for example sending only the changed value respect the previous one, etc...) 3) To develop and give me the source code, to be imported in Android Studio, in order to show the map with the following functionalities The developper could use whatever open source code and/or jar library already existing. Most of the functionalities I'm requiring are already available for free on internet, but I need an expert that suggest me the right choose and write the code according to my specs. See enclosed file "Specs for AVL [login to view URL]" for complete specs. Payment Milestone 1° Milestone - delivery of first release with BASIC functionalities - 50% of total amount 2° Milestone - delivery of final release with ADVANCED functionalities - 50% of total amount