Yes—if the person and the tree are at roughly the same distance from the camera and standing vertically in the same plane, you can use a simple scale ratio.

Formula: unknown height = reference height × (unknown image height / reference image height)

Example: If the person is 400 px tall and is 1.8 m in real life, and the tree measures 900 px in the image: 1.8 × 900 / 400 = 4.05 m

You don’t need a special app: you can measure directly on screen with a ruler after zooming, or use image tools that report pixel distances.

Tools mentioned by the community:

• GIMP: set the image scale using the known reference, then use the Measure tool in physical units.
• Meazure: for measuring pixel dimensions.
• Blocklayer Scale Fixer: an online scaling tool.

Important limitation: this only works well when both objects are at the same distance from the camera and perspective distortion is minimal. If the tree is farther away or the camera angle is strong, the estimate can be inaccurate.