## 4 Discussion of the Results Analysis: First Findings on Trends and Best Practices
### Main Findings for the Grey Literature Sample
#### 4.2.1 Aerial Photography: Commonalities and Regionalsisms  
With regard to \"Aerial Photography\", the use of aerial documentation, either vertical or oblique, from either historical collections or new surveys, is fairly common to northern (e.g., England, Scotland, Ireland, Denmark), central (e.g., Germany, Poland), and southern (e.g., Italy, Spain) European countries. Nevertheless, it is to be noted that a detailed discussion of the different typologies of aerial photographs, and of how to undertake photointerpretation, is found only in the Grey literature documents that are issued by heritage bodies and organizations that have established technical expertise and/or in-house services or departments for the collection, storage, processing, and dissemination of aerial photographs. For example, all the documents issued in the northern countries included in the analyzed sample fall into this category. Instead, more generic mentions are found in the other documents analyzed. In some instances, documents include technical considerations that highlight a higher level of awareness, by users and practitioners, of imagery that is collected in other wavelengths of the electromagnetic spectrum than merely the visible bands. For instance--albeit referring to satellite imagery, but the concept is basically applicable to aerial photographs too--the Short Guide on applied digital documentation in the historic environment [32] issued by Historic Environment Scotland (2018) mentions the existence of multispectral sensors including additional infrared bands that _\"can provide further information about the surface captured, identifying otherwise hidden features, helping to 'classify' areas (e.g., as urban, water or vegetation) and even showing emitted thermal radiation\"_. More detailed is the discussion by [PERSON] [33], who highlighted the sensitivity of the infrared portion of the spectrum to changes in vegetation and the consequent benefits for the detection of subsoil archaeological remains. The author also recalled that _\"the infrared spectrum has been shown to be particularly useful in identifying and interpreting coastal features and assessing vegetation type and health (a stronger red/orange hue is related to vegetation health and growth)\"_. However, in [33], the author also acknowledged that, at that time, infrared imagery was not used very commonly. While this statement may nowadays seem to be obsolete given the time-lapse from when the document was published, it actually finds matching evidence in the fact that very few of the Grey literature documents explicitly mention infrared aerial photographs. Even less is the interest in thermal imagery. Again, among the analyzed Grey literature, [PERSON] is the sole author to comment on the usefulness of thermal imagery for archaeological applications [33]. Therefore, the final impression is that, apart from some exceptions (particularly when the level of available technical expertise is highly specialist), the use of aerial photography is mostly confined to imagery collected in the visible and, secondarily, in the near-infrared bands. One potential limitation that may occur if only technical recommendations and guidance documents are analyzed is that such Grey literature is by definition programmatic. Therefore, it does not necessarily provide sufficient evidence that the best practices and technologies recommended therein are effectively implemented and followed by practitioners and officers and thus exploited in daily practice. In this respect, better evidence can be found in management plans. For example, the Management Plan of the UNESCO World Heritage List Archaeological Area and the Patriarchal Basilica of Aquileia [50] explicitly lists photointerpretation of aerial photographs (either already available or collected on purpose) among the operations needed to prepare the required documentation to implement the safeguarding of areas of archaeological interest.