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.