The paper presents a study on the determination of soil moisture using the so-called triangle method which is based on analysis of the scatter plots of LST (Land Surface Temperature)  and VI (Vegetation Index). On the basis of LST-VI scatter plot the called temperature-vegetation drought index, TVDI (Temperature-Vegetation Dryness Index), can be determined, which is a good measure of the soil moisture available to plants. However, determination of such an index can be carried out in several manners and using different vegetation indices. The aim of the work was to present and compare three types of method for determining the TVDI index using two different vegetation indices, NDVI (Normalized Difference Vagetation Index) and EVI (Enhanced Vegetation Index), with particular emphasis on determining the so-called "dry edge”. The TVDI drought index was determined for the Kampinos National Park area. The authors obtained interesting results, consistent with the literature. In this paper the authors present in detail, and compare with each other, three methods of determining the TVDI index using two vegetation indices - NDVI and EVI, with particular emphasis on determining so-called "dry edge". In addition, a detailed statistical analysis of the results obtained by using three modifications of the triangle method was performed. It is worth to mention that soil moisture, and thus the measuring methods, are important from both the scientific and the practical point of view (e.g. risk of fire, agriculture).