Refraction Correction for Connection of the National Leveling Network First Order to Tide Gauge Station Varna

The paper outlines some problems when applying refraction correction in order to reduce the effect of vertical atmospheric refraction in precise leveling observation. An example for calculating the refraction correction for a leveling line that connects the National Leveling Network first order to tide gauge station Varna is given. The estimation is using the model described in new Instruction № RD-02-20-1 of 15 January 2021 for the Establishment and Maintenance of the National Leveling Network. The results show that that the refraction correction is commensurable with correction for rod scale and must be applied for each set-up, appropriate weather conditions when the air temperature changed slightly throughout the day is better to be chosen. The applying of the refraction correction does not eliminate the requirements related with the maximum length of sight, minimum high of sight, balancing of the length of sights, and the choice of appropriate weather conditions.