Boundary paintings, or jie hua, is a traditional Chinese painting style originally created for architectural design. Today, boundary painting is valuable for historic conservation since it provides accurate depictions of heritage gardens. However, this valuable evidence has not been fully decoded and used in conservation practices, because there is no effective method to translate these artworks into spatial information that can be directly applied. Therefore, the aim of this research is to demonstrate how to use 3D mapping technologies to decode boundary paintings and translate them into spatial indicators and parameters for spatial conservation of heritage gardens. This approach will be explored through building 3D virtual models of heritage gardens based on the spatial information extracted from historical boundary paintings. Some indicators, such as visual openness, connectivity and intelligibility, will be explored in the process to represent spatial patterns of heritage gardens. At the same time, aerial images of specific gardens will be captured from existing environment using UAV in accordance with the viewpoints in boundary paintings. These images will be compared with boundary paintings of the same garden to confirm spatial patterns and to detect changes. The outcome of this research will be a staged guide to decode boundary paintings and provide indicators for spatial pattern conservation in heritage gardens. This guide will be interested by landscape conservators, heritage planners and digital program developers.