## 4 Quality Control  
Quality is defined as the \"property inherent in anything that allows it to be compared with anything else of the same species or nature\" [14]. For this purpose, certain procedures and controls have to be provided to guarantee the integrity, accuracy and entirety of the data. In other words, it is necessary to verify that the desired quality has been reached [14].  
In the case of the information provided by the photogrammetric method, these are the classic aspects [15] related to redundancy measures. The results of the photogrammetric triangulation provide the quantitative measurements for the precision of the results:  
* Variance of the components;
* Variance-covariance of the coordinates of the calculated objects;
* Comparison of values with nominal data;
* Independent measurements to verify the precision via a point control analysis; and
* Comparison of coordinates of the photogrammetric points with the independently obtained coordinates (i.e., GPS in the field).  
With regards to the LiDAR information, the control is a secondary procedure to ensure and verify the quality of the registered data. It can be conducted in terms of two criteria:  
1. \"Causes\": study the behavior of the elements that define the mathematical model of adjustment, where we consider both \"internal causes\" (flight plan: external orientation and calibration) and \"external causes\" (flight conditions: direction of the flight paths, flight height, etc.) as well as ground type (altitude, type of vegetation, etc.).
2. \"Effects\": the results given by the point cloud and where the following has to be taken into consideration: the \"relative internal effects\" (topographical control and planimetric control) and \"relative external effects\" (control points, difference between Digital Terrain Model and DSM, differing intensity levels and stereoscopic checking). In most cases, the quality control of the results is conducted starting with this second criterion, while the possible corrections of the results are completed using the elements of the equations chosen for the adjustment.  
In our case of study, we performed three tests to estimate the quality of the results obtained (a test zone in the city of Alicante, Spain) with the software developed. First, a planimetric control was used (only to estimate accuracy for \(X\) and \(Y\) coordinates). In this case, we compared a picture obtained with classical photogrammetry PNOA (Plan Nacional de Ortofotografia Aerea) [16] with a synthetic picture generated for the same place using our methodology (we describe this procedure in planimetric analysis, Section 4.1). The PNOA picture has the best accuracy so this is used as reference. For the second test (to estimate accuracy for \(Z\)-coordinate, described in Section 4.2 as 3D analysis), we used a similar procedure to compare two digital models. The first digital model was obtained from classical photogrammetric pictures, while the second one was obtained from synthetic pictures (both in the same area, Alicante, Spain). Finally, we compared these two images with \(X\)-, \(Y\)- and \(Z\)-coordinates using a 3D transformation. In order to compare all these data and to measure the quality, we have used the software developed within the framework of a previous study [14].