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The authors present the results of their research into photogrammetric documents based on synchronic digital images. Examples presented in the paper indicate the value of synchronic stereophotography in registering phenomena connected with liquids. Combining synchronic stereophotography with photogrammetric methods has allowed the authors to develop an application useful for monitoring the flow of a liquid in an open channel. The paper is supplemented by the scientific approach to the development of a stereo pair using a proprietary algorithm and data elaboration. The tasks performed allow an assessment of the accuracy of the results obtained and indicate significant usefulness of photogrammetry and synchronic photography methods in liquids measurements and monitoring. The problem concerning the measurement of the liquid velocity in open channels is an important issue for the calculation of flows volumes. The open channel can be understood as both open riverbeds and channels and each duct with the free surface of the liquid. In the context of the application of the described method, the open channel concept can be extended to the waters released as a result of the failure of hydraulic engineering devices and flood waters occurrence. Liquids in the open pipes are limited with channel walls, whereas they surface-border on the air and form a free surface of liquid. The liquid flow is not subject to pressure and it is related to the fluid properties and the levelling decrease of the bottom, roughness of channel walls and the gravity. The velocity distribution in the horizontal and vertical section is not uniform and the equations used to determine the average velocity and the velocity distribution are empirical or semi- empirical formulas. Therefore, the choice of measurement and computational method is significant for the achieved results (1). Methods of the measurement of the flow velocity, used in open channels: • velocity-surface method (using a hydrometric grinder or float); • declivity-surface method; • index dilution method; • hydraulic methods (swelling and reducing); • ultrasonic methods; • optical methods (using laser Doppler anemometry or PIV, i.e. Particle Image Velocimetry); • electromagnetic methods (2). Due to the difficulties with the flow modeling in open channels and the large impact of the weather conditions (wind, ice) (1), the float method gives the largest measurement unreliability. In the hydrology, the surface measurement is considered as indicative. At the same time, this method is cheap and easy to implement. It consists in measuring the surface speeds with the usage of floats on the chosen section of the open channel. Conditions relating to the procedures have been standardized (ISO). Basically, this method is applicable in case of unavailability or difficult installation of hydrometric equipment, as well as the confluence of ice floes, flows of the great waters at very low water velocities and for the purpose of the long- term observation of the water mass movement, e.g. in lakes. Measurements with the use of floats have been known for

Synchronic digital stereophotography and photogrammetric analyses in monitoring the flow of liquids in open channels