## 1 Introduction
As an example, the ASPRS standard [12] indicates that well-defined points should be easily visible or recoverable on the ground, on the independent source of higher accuracy, and on the product itself. This supposes that checkpoints should contain certain characteristics that make them geometrically particular, avoiding other points that are not so easily identifiable. Most of the studies that have analyzed the accuracy of DEMs used checkpoints based on Ground Control Points (GCPs), which are obtained in the field using surveying techniques, such as Global Navigation Satellite Systems (GNSS). The use of GCPs influences their distribution because they are usually located in accessible areas in order to reduce field costs. To avoid the use of GCPs, this study focuses on the use of another, more accurate DEM as a reference. The use of this reference adds another issue related to the differences in the spatial resolutions between the reference DEM and the destination DEMs and their repercussions on the identification of checkpoints. The manual selection and measurement of checkpoints supposes a great deal of time and cost effort for any affected project, depending on, for example, the sample size of the points to be used. In addition, this manual selection is conditioned by the operator's experience. We must consider that two different operators will select different checkpoints following their own criteria, causing a certain bias. The use of an automatic procedure will avoid this problem because the selection will follow certain criteria, giving the process greater repeatability. Considering the aspects described previously, an automatic procedure to determine checkpoints from both sources (to be assessed and referenced) is desirable. The use of an automatic procedure will provide a cost reduction and an improvement in the assessment due to the increase in the sample size by covering the DEM with a larger density of checkpoints. In addition, the automatization would remove the biases caused by operators. On the other hand, this assessment should analyze both vertical and horizontal accuracies in order to study the influence of the horizontal discrepancies in height values. In this study, we propose the possibility of using a set of checkpoints obtained directly from both sources and, more specifically, from two DEMs (the one to be assessed and the reference one). To achieve this objective, it is necessary to develop a new methodology that allows the automatic detection of homologous points from both sources but also considers the characteristics of DEMs (spatial continuity, low variance, etc.) and the necessity of using well-defined points. As mentioned previously, the ASPRS standard [12] establishes some conditions applicable to checkpoints to be used in the positional accuracy assessment. These checkpoints must be clearly identifiable, both vertically and horizontally. Therefore, these aspects must be taken into account when developing this automatic procedure. The automatic detection of homologous points contained in several images has been extensively studied during the last few decades. Several procedures based on point detectors have been described for their use in computer vision and photogrammetry, such as the Scale Invariant Feature Transform (SIFT) [15] and the Speeded Up Robust Features (SURF) [16]. Some recent studies have compared these detectors [17; 18], and some applications have been performed with DEMs based on these algorithms, such as those oriented to the registration of several datasets [19; 20]. Although these algorithms are focused on obtaining interest points that are common to several images, and these homologous points could be used to compare their coordinates, most of them are not suitable for a positional accuracy assessment because they cannot be assumed as well-defined points (considering the conditions indicated by the standards). Obviously, some of them could meet the requirements.