Providing users with carefully authored Locative media experiences (which can be consumed via their GPS equipped smartphones or tablets) has significant potential for fostering a strong engagement with their current surroundings. However, the availability of mobile tools to support the authoring of locative media experiences in-situ, and by non-technical users, remains scarce. In this article we present the design and field-trial expert evaluation of a mobile app developed under the SHARC project (Investigating Technology Support for the Shared Curation of Local History in a Rural Community). The app is named SMAT (SHARC Mobile Authoring Tool) and supports the authoring of Locative Media experiences with a focus on the creation of POIs (Points of Interest) and associated geo-fences which trigger the pushed delivery of media items such as photos, audio clips, etc. One important requirement of SMAT is the ability to support authoring in places where connectivity is intermittent or unavailable, e.g. many rural areas.