Method for measuring of a basal area of trunks in a forest, said method being based on decoding photos taken of a forest cover, wherein the method comprises steps of: - using a camera , whose horizontal view angle value, α, is known and is kept constant when taking sample photos for measuring, - taking sample photos by means of the camera of the forest cover in an area to be surveyed, wherein measuring data is included in each sample photo , - storing the taken sample photos of the forest cover into a database created in a web server, - calculating, in a web server, the number of trunks from the sample photos taken of the forest cover, wherein such a trunk whose breadth as pixels exceeds a limit value of pixels measured for the camera is calculated to add to said number, - when the horizontal angle value, α, of the camera is smaller than 360°, multiplying the number of trunks by a value 360°/α, - carrying out election of the trunks for the calculating of the number of trunks and defining their tree species by a person or group of persons using a web browser connected to a cloud service, wherein, in the web server, a person enters via the web browser to look at the sample photos , identifies the tree species on the display , picks up trunks for trunk calculation and stores the measurement, the trunks that can be accepted as sample trunks being elected, from the sample photo surveyed on a display , based on a measurement of a breadth of said trunk utilizing a line segment moved on the display or a movement of a cursor from one side of the trunk to another, and - determining the basal area or basal areas per tree species of the forest cover in the surveyed forest area per area unit from the number of the trunks . - using a camera , whose horizontal view angle value, α, is known and is kept constant when taking sample photos for measuring, - taking sample photos by means of the camera of the forest cover in an area to be surveyed, wherein measuring data is included in each sample photo , - storing the taken sample photos of the forest cover into a database created in a web server, - calculating, in a web server, the number of trunks from the sample photos taken of the forest cover, wherein such a trunk whose breadth as pixels exceeds a limit value of pixels measured for the camera is calculated to add to said number, - when the horizontal angle value, α, of the camera is smaller than 360°, multiplying the number of trunks by a value 360°/α, - carrying out election of the trunks for the calculating of the number of trunks and defining their tree species by a person or group of persons using a web browser connected to a cloud service, wherein, in the web server, a person enters via the web browser to look at the sample photos , identifies the tree species on the display , picks up trunks for trunk calculation and stores the measurement, the trunks that can be accepted as sample trunks being elected, from the sample photo surveyed on a display , based on a measurement of a breadth of said trunk utilizing a line segment moved on the display or a movement of a cursor from one side of the trunk to another, and - determining the basal area or basal areas per tree species of the forest cover in the surveyed forest area per area unit from the number of the trunks . <EOS>