## 6 Surface Characteristics Assessment
### Surface Texture Assessment
#### 6.2.2 Non-Contact Methods  
Non-contact methods can be used for the evaluation of the macrotexture of pavement surfaces and some methods have greater precision than others, meaning they can also be used for microtexture measurement. These methods include image texture analysis [123; 124], stereoscopy [125], computer tomography (CT) scanning [126], three-dimensional (3D) scanning [127], and laser profilometry [98]. All of these methods require precise and expensive equipment, with some of the methods only being able to be performed in the laboratory, and some requiring long test times [128]. This limits these methods to research activities and it excludes their use in routine runway surface texture assessments. ICAO allows for the use of profilometry as a tool for macrotexture assessment [7]. During the profile assessment, the mean profile depth (MPD) is calculated, which can be converted to an estimated MTD in most cases. Profilometry estimated MTD usually differs from the volumetrically measured MTD because of the test methods, with volumetric methods affected by any deep pores in the material. For open graded courses, also known as porous friction courses, this difference can be significant and needs to be considered during the measurement [106]. In contrast, for dense graded asphalt and concrete, the two methods provide more comparable results [80]. There have been various attempts to develop a universal correlation between the volumetric and profilometry-based methods. However, different studies have reported different results. For example, during the PIARC experiment performed in Europe [104], a correlation between the two methods was developed. The correlation was used in different standards, such as [129; 130]. However, subsequent studies reported that this correlation plot cannot be reliably used for the assessment of MTD [106]. For example, Figure 25 compares the PIARC correlation with measurements from independently tested surfaces. The difference is clear, particularly at MTD values above 2.0 mm. In another study [80], the laser texture method was used to compare the conventional volumetric method of the MTD calculation to laser profilometer-based estimates of MTD. Figure 26 shows the correlation between MTD and MTD for all surfaces, which is clearly non-linear, as stated in the PIARC model. Furthermore, Figure 27 shows the improved correlation between a non-linear (quartic) conversion from MPD and an estimated MTD (referred by as ETD), which is consistent with the findings in Figure 25.  
Figure 25: Correlation between mean profile depth and mean texture depth [106]. It is still a debatable question of which parameter is more representative, mean texture depth or mean texture profile, since these two methods represent two different properties of a surface, both of which are important for different elements of skid resistance assessment. The MPD estimates the braking force and drag available to the tire more precisely, because laser techniques are assessing the effective contact area between the pavement surface and the tire by omitting the effect of deep pores in the surface. In contrast, the MTD estimates the ability of the pavement to drain water from the pavement-tire interface, which is an important element of aircraft skid resistance in wet weather and is the focus of aircraft skid resistance management. Given that the two measurements focus on different surface characteristics, it is no surprise that a universal correlation between the two has not been found to be applicable to a broad range of surface types. Figure 27: Correlation between estimated MTD (ETD) and measured (sand patch) MTD for different pavement surfaces [80]. Full 3D imaging laser technology can also be used for macrotexture assessment as well. However, the area-based assessment, rather than a profile, allows for microtexture to be estimated as well as macrotexture if the precision is adequately high. For example, a low-precision scan was used to assess the macrotexture parameters of various surfaces [131].