Systems, methods, and graphical user interfaces for annotating, measuring, and modeling environments

A computer system displays an annotation placement user interface that includes a representation of a field of view of one or more cameras that is updated over time based on changes in the field of view, and a placement user interface element indicating a virtual annotation placement location. While the placement user interface element indicates a first location in the representation of the field of view, in response to receiving a first annotation placement input, the system: displays an annotation at the first location in the representation of the field of view; and displays one or more first guides that extend from the first location in the representation of the field of view.