Ground truth data is essential for reliable and accurate agricultural research. For example to monitor crop productivity, inputs such as crop location, biomass and yield are required. To train and validate the algorithms and models to produce such inputs, researchers rely on ground truth data, which is not always available. Published and open data could be used to fill this gap, yet these data are scattered over many different sources, lack standardization and have incomplete metadata. This hampers the re-use of this data by others, causing an inefficient use of resources, while also limiting the calibration and validation work which in turn affects product quality. To address this problem AGROSTAC was initiated by Wageningen University (WUR) and Research and Vlaamse Instelling Technologisch Onderzoek (VITO). AGROSTAC collects and harmonizes georeferenced open data around key agronomy observations such as crop type, phenology, biomass, yield and leaf area. Published, open data sets are screened for key observations and selected data goes through a dedicated data curation procedure. This is a crucial step to ensure re-use of data beyond its original purpose of collection. In this procedure, metadata are checked and completed using all information available in the data files, supporting documents and associated publications. Data are converted into standard units, and phenology events are mapped to the BBCH scale. As a result, data can be offered in a FAIR manner (Findable, Accessible, Interoperable, Reproducible), and ready for use. This publication includes two documents: 1) a presentation on AGROSTAC and 2) a description of the AGROSTAC APIs interface to access and use harmonized datasets in separate workflows. To ensure the proper re-use of the harmonized data you must check the data license of the original data set. An overview of the harmonized datasets and metadata such as data license, citation, original data holder is available via this link (AGROSTAC API request 3).