PHOTOGRAMETRY TIME. A CLASSIFICATION PROPOSAL FOR PHOTOGRAMMETRIC SURVEY TIEMPO DE FOTOGRAMETRÍA. UNA PROPUESTA DE CLASIFICACIÓN PARA EL LEVANTAMIENTO FOTOGRAMÉTRICO

The graphic survey of the built heritage allows and makes possible a graphic territory of learning, analysis, experimentation, communication and deposit for architecture. It supposes, in addition to the connection between the theory and the practice of our discipline, an excellent framework for teaching. In this context, photogrammetry is evidenced as a rigorous and efficient technique that, at the service of relevant hypotheses, is capable of producing results that are as valuable as they are contemporary. This research paper, created after the tutelage of more than forty final degree projects that use the photogrammetric technique, develops a classification proposal for qualified point clouds (as part of the technique workflow) based on the temporal nature of the drawings. In this way we will argue the three possible positions (previous or tool, intermediate or process and later or result) to conclude by claiming the relevance and importance of their consideration during the value of the drawings of the built architecture.


INTRODUCTION
The graphic survey of the built heritage allows and makes possible a graphic territory of learning, analysis, experimentation, communication and deposit for architecture. It supposes, in addition to the connection between the theory and the practice of our discipline, an excellent framework for teaching. In this context, photogrammetry is evidenced as a rigorous and effective technique that, at the service of relevant hypotheses, is capable of producing results that are as valuable as they are contemporary. Our strategy, based on the temporal nature, relates drawing with the active and investigative process that, in one way or another, requires and generates it.

STARTING HYPOTHESIS AND GOALS
We start from two hypotheses: first, the graphic documentation is a tool to achieve a goal that is not necessarily the creation of this documentation and, second, heritage survey in general (and photogrammetry in particular) is part of the elements of a process that must be oriented, first of all, by the intentions and purposes that generate it. The main goal, continuing with this idea, is aligned with the need to test the proposed strategy, derived from the previous hypothesis and synthesized in the lines that follow in the form of a taxonomy. It will be carried out through the analysis of the graphic results and, finally, the scope, repercussion, effectiveness are valued and an exportable criterion is established; this allows the researcher to choose the best method of working according to the nature of their drawings and their positioning regarding to the temporal.

METHODOLOGY
In the first place, we will explain the basic process carried out (repeated in all the supervised degree projects that incorporate the photogrammetric technique) and, later, we will divide the work into three stages. In the first place, the subject is focused from a theoretical point of view to establish the series of axioms, hypotheses and previous reflections (already announced above) that make up the starting point of the investigation. Second, the analysis of the graphic database made up of research that uses drawing as an essential tool is presented. Finally, thirdly, the most effective strategies (and their consequences) for the enhancement of architectural drawings are discussed.

COMMON TOOLS, METHODS AND WORKFLOWS. THE STARTING POINT TOWARDS DIVERSITY AND GRAPHIC INNOVATION IN AN ACADEMIC CONTEXT
Although the software (or software) and the physical (or hardware) are, by definition and necessity, updated over time or, what comes to the same thing, permanently modified, we consider it appropriate to record and specify those used during these ten years of study. In this way, and collaterally, we will unravel the methods and workflows that students (among whom we paradoxically include ourselves as teachers) have been developing.
- Tools. As we say, the current minimum requirements are constantly changing 1 . In any case, the issue is, in most cases and if the program is compatible with the operating system, a longer wait during calculation and automation. In our specific department (Graphic Expression, Composition and Projects of the University of Alicante) we have had a professional license for the Agisoft Metashape 2 program for a few years. Also with the Nikon model D90 camera and an extendable support (pole 3 ) for lifting it up to a height of 9.27 meters ( Fig. 1). Likewise, the drone that we usually rent (in those case studies that require it and allow it, since both are equally important conditions) is the Phantom 3 Pro 4 . However, and despite the fact that we can say that these are our basic tools, students are able to use and incorporate many more, both physical (different types of drones and reflex cameras) and logic (various computer programs that allow measurement and virtualization from photographs, such as RECAP or sketchUp, to name just two). - Methods and workflows. In general, the process involved in photogrammetry remains constant. The recent advances in the development of the programs allow an automatic calibration of the photographic camera so that the most important step of the process can be said that it happens during the taking of snapshots. To a large extent, the success of the derived steps will depend on them, which, in most cases, are the following (all of them already in virtual and three-dimensional space): orientation of the cameras, calculation of the dense point cloud, mesh model and textured model (Fig. 2).
However, the research, once we have exhausted the workflow in the specific software that we are using, continues (as we will see below) depending on the intentions, starting hypotheses, conditions and limitations of the author. The transversality but, above all, the absence of a definitive program with which to start, process, and finish the photogrammetric work, will make us export and import the model in three-dimensional rendering and visualization software (such as Lumion), modeling and modification (like Rhinoceros, AutoCad, Revit or Skecthup, for example) or image editing (like Photoshop or GIMP2) to name three possible graphic paths. The important thing will be, as we will argue, the new order of things in which reality is represented from the first and most important of the photogrammetry works: the one in which we capture images.
- Starting Point. The proposed generic methodology ends here. Previous studies, such as that of Jorge Sainz, allowed us to establish a first way of approaching the characteristics of each student's drawings "Of the three dimensions that we have established for the study of drawing -use, way of presentation and graphic technique-" (Sainz 1990: 77). Our work began at this point since each of the students, each of the final degree projects, had to find their own way of shedding the necessary light on the objectives set. Precisely one of the most important reasons and that supposed the beginning of this research was to find the common guidelines to the evident diversity that we faced in the results of the research. Some works used web repositories as an approach to the visual experience, others investigated the use of augmented or virtual reality through innovative processes and tools, there were also those that created interactive maps or, simply, understood as the only possible end for the drawing of heritage built to the traditional (and effective) dihedral projections. From the need to organize this diversity arose the classification proposal for the photogrammetric survey that we will describe below.
The contemporary drawing of architecture, understood as a static and two-dimensional representation of the built space or, more generically following Javier Seguí as "the product of drawing" (Seguí 2018: 63), has been incorporating the set of tools used by the advances latest technology. These, together with the more traditional techniques, are those that define the territory of possibilities that the two-dimensional architectural graphic expression has within its reach. In other words, there are undoubtedly now more and better tools to graphically document heritage. Also to represent imagined architecture. However, we must underline the fact that by using contemporary technology we architects are able to establish a new three-dimensional database (in some cases called the digital twin). Whether in order to obtain the usual static and two-dimensional drawings or not -we remember that each drawing responds to an intention, which we must explore to interpret its qualities (Montes 2017: 13) -, the truth is that this new digital imaginary where depositing the information in a three-dimensional way also allows us to interact with the database following the rules and conditions of the ways of access to it or, in other words, the software that allows it.
It is in this context where the photogrammetric database arises, which begins with photographs, but it is synthesized in the point cloud, as a new model on which to develop knowledge and to draw. That is, once the field work has been carried out and the photographic captures have been completed, the attention shifts from reality to the new database (starting with the oriented photos and the first automatically generated point cloud), which acts as a new reference point for the drawing. This fact, shared with the application to drawing of other technologies such as the laser scanner (already included by default in the new models of Apple smartphones), we believe is unprecedented in the history of drawing. The signifier goes from being the real world to becoming evident as its digital reflection. The architecture itself can continue to be revisited and analyzed, but the effort is now based on elaborating knowledge from this second derivative that spatially oriented photographs suppose (in an aseptic three-dimensional model and still empty but georeferenced) and in the point cloud that consequence. It is for this reason that we say that we have gone from the line to the point (Juan & Marcos 2016) since, from contemporary times, with the help of computers, the points and their management will be the true protagonists of the germ of graphic expression... occupying a role that, until now, corresponded to the line.
The process that allows the suggestive world of drawing can also be understood as the series of Russian dolls in which the purpose of each of them ends in opening the next one. The stages of drawing begin and end in the definition of its limits and synthesize the series of efforts that, around the visual, organize the databases of each of the stages or moments. The goal of drawing is to be able to draw. In this context we can affirm that the cause of the drawing is another drawing. Starting with the representation that, by inhabiting the world, we become of it, and ending with the paper support with the series of ink modifications that represent a built space, the role of the drawing is to serve as an aid or tool to understand and communicate and it is in this sense that most research uses the graphics: as a lever to develop their objectives.

AFTER THE PHOTOGRAMMETRIC SURVEY. STRATEGIES FOR THE ENHANCEMENT OF ARCHITECTURAL DRAWINGS, BUILT OR NOT
The common denominator of the architectural heritage research exercises (which have graphic expression as the central nucleus of their analysis and with photogrammetry as a technique) is, in most cases, three-dimensional modeling (point cloud plus mesh plus textured). But the proposals do not end in the drawing as a document in itself since what makes them constitute an investigation in an architectural context is the establishment, first, of starting hypotheses and their own objectives to, later, develop a methodology that, based on graphical models, allow to establish some concussions in view of the results. Although the direction and structure is common, each of the proposals will be different not only in the way they approach the exercise but also in their own previous hypotheses and approaches. Next, we will summarize the main ones; We have classified them into three types, according to their temporary position at work (before, during or after), namely: drawings as a tool, drawings as part of the process and drawings as documents.
- Drawings -tool (or before): Understanding the drawings as a tool implies the need to obtain them prior to the development of the methodology itself. As Javier Ortega so aptly reminds us "the drawing can be proposed as a research purpose, considered in itself, or be used as an instrument of the same to progress in the knowledge of the built heritage" (Ortega 2016: 296). The initial hypotheses of the work require graphic documentation as part of the instruments necessary for testing it, and our threedimensional models, the result of the calculation that photogrammetry entails, will be claimed, first of all, for their usefulness.
- Drawings -process (or during): The graphic part of the investigation can also be understood as the verb that involves using the drawing as a useful action that, like scientific experiments, underlines a search, a comparison, a test but, above all, an interaction with the author of the work. The graphic is, in this way used, a means of research in permanent change and, therefore, it is not so much about what is achieved with it (tool) or what it supposes as an inert element (document) but about what it is able to offer during the derived dialogue exercise its construction line by line or, as we would say today, click by click. This causal action of drawing, understood as a process in itself, is what Berger refers to when he affirms that "drawing in order to discover, this is a divine process; it is to find the effect and the cause" (Berger 2005: 66). Or to which Giandebiaggi refers us when, citing de Rubertis, he reminds us that "the drawing serves as a temporary support of the mind and imagination" (Giandebiaggi 2016). Only through the action of drawing (and what it entails) are we capable of thinking, reasoning and, therefore, scrutinizing and assessing what the different possibilities that we ponder when we work and analyze regarding heritage entail. There are numerous authors who abound in the idea that drawing (manual, technical, assisted, photogrammetric,...) fulfills a role beyond the instrumental with respect to the representation of architecture. In the opinion of Otxotorena (1996: 89) the conception drawing is closely linked to the graphic medium in which it develops. The drawing becomes the proper and specific instance of the projective discourse: that is, the conductor and the medium of what has come to be called "architectural thought" and the medium of creation.
- Drawings -document (or after): As a result of the methodology and in order to shed light on the conclusions, the drawing can be analyzed, created and also understood as a final document. The value of the graphics, in the latter case, is intrinsically linked to the possibilities of said element as a database itself. The use of the photogrammetric survey as a document is part of Dernie's affirmation: "drawing and virtual images are the key to the coherence and integrity of the design, since they are the only elements that can express the relationship between the concept or the intention of a project and its material form"(Dernie 2010: 8).

APPLICATION OF THE PROPOSAL. SCOPE AND LIMITATIONS: POINT CLOUDS AND ELLIPSIS
The final degree projects that are analyzed in this section have the common denominator of working with a fundamentally graphic basis to achieve the objectives. Either through the use of photogrammetry or with another series of technological tools, the hypotheses of all of them require drawing to be achieved. We will graphically illustrate each of the proposed epigraphs below, although, remembering Otxotorena's words, we must indicate in advance that the containers are not watertight and there are many results that can be understood differently depending on the stage in which they are evaluated. " 'Drawing as an end' and 'drawing as a means' represent different horizons; in principle complementary. And there is something to understand as soon as possible, to avoid confusion: the type of drawing that architects practice in itself is halfway between them." (Otxotorena 2016: 55) The database as a tool (Fig. 3): Understanding the point cloud first, and all its derivatives, later, as a tool allows us to have an additional research instrument for our work that, well applied, will undoubtedly help to achieve the objectives and the verification of the starting hypotheses. In this way, clouds of millions of points, three-dimensional geometric models, tile models and all the possible combinations and operations that allow a replica of the discretized reality in positions and colors, are evidenced as starting points of possible graphic paths. In Fig. 3 we see an example that, by means of a fragment (in the form of a slice parallel to the horizontal plane) sectioned from a cloud of points, helps to establish a comparison between the drawing of reality (vector planes) prior to the photogrammetric survey and reality itself in the form of a point cloud (with a precision, remember, of plus minus 1 centimeter).
The database as a source of knowledge ( Fig. 4): In this second example we start from the graphic documentation (accessible only thanks to the benefits of photogrammetry) to elaborate knowledge and additional information that, in addition to being unpublished, will be implicitly linked to the database from which it starts. Only possible in a graphic discourse the series of approximations and hypotheses will be graphed as part of an interactive process. We can observe, in this case, how the student geometrically rehearses the series of theoretical approaches in order, precisely, to check measurements, textures and hypotheses and to be able to vary the drawing according to the results.
The database as a document (Figg. 5 and 6): Perhaps the most traditional (and no less efficient or contemporary) way of approaching the drawing of architecture through photogrammetry is to generate documents that, as if from a deferred architecture (Ortega 2016: 296) will be involved, they will form part of the theoretical body of the same and will give testimony, like this one, of the material spaces prepared to inhabit. At the same time they will serve as a repository of memory and testimony of what, in a very specific place and time, their physical and material reality meant. Figures 5 and 6 are examples of this: contemporary drawings understood as documents faithful to a discourse and highly precise in terms of the geometry they treasure.

RESULTS
As we have seen, and in the same way that happens during the exercise of the profession, the drawings in particular and the graphic expression in general offer us, precisely because of the images they relate, a promise of duration, of permanence against the passage of time (Brea 2010: 9). In addition to being shown as fabulous work tools, the drawings help us to think and reach theoretical places that would otherwise be deserted. Even the graphic results of the workflow that photogrammetry implies (in theory aseptic although with the particularities that the different conditions for capturing snapshots impose) can be understood, and treated, as generators of critical and partial readings, that is, as tools, processes or documents (following our classification) of incalculable value in an innovative and research context (Fig. 7). As the Puche Fontanillas work team rightly reminds us "We intend, beyond its geometric reality and its reflectance values, to extract further semantic content from the visualization and / or management of the point cloud" (Puche 2017: 229).
But the drawings also show, at times, our limitations. Let us remember the words of Antonio López quoted by Óscar Tusquets "You don't know how the sun used to gild the leaves and the upper quinces this morning. It was beautiful, but, of course, impossible to paint" (Tusquets 2019: 47). The dialogue (first interior and, later, exterior) that allows the drawing process supposes a work methodology in itself that, many times and even in the absence of connections with the construction or the project, we have demonstrated fertile and effective. As if this were not enough, the final result, understood as a document, is part of the architectural heritage in the same way as the physical reality itself that we call architecture. The drawing can also be included as architecture and, as such, it must be (when required) conserved, analyzed and put in relation.
Although we have tried to guide and classify them (taking into account their use of graphics), the diversity in the use of graphics in academic research is overwhelming. Once they have positioned themselves to articulate the architectural drawings (around which they orbit at all times), each of the authors demonstrates the high capacity and possibilities that graphic language possesses interacting with the visual in a decisive way for the results of the personal process and the achievement of the hypotheses to which we referred above. The consequence of all the above is a set that can hardly be circumscribed to a specific field of knowledge: the research that use the photogrammetric technique cover a field as broad as that of the discipline itself that uses them both for measurements and

CONCLUSIONS
Although, as has already been indicated, the material and the database with which we have worked far exceed the light that we can throw in the present conclusions, we can try to synthesize the results taking into account the temporal structure of the classification in the first place. yes, secondly to its relevance and scope and, thirdly and lastly, to the application of the photogrammetric technique itself in our context: - The positioning in relation to the temporality of architectural drawings allows us to identify three common places, namely, tool (before), process (during) or document (after). This strategy to understand the graphic is inevitable (all drawings have a place in it) although not exclusive (the - Although it is a first classification that we have proven useful, the nature (scope, impact and, as a consequence, the final value) of each of the final degree projects depends, to a large extent, on the relevance of the starting hypotheses raised and the rigor of the methodology used, and not so much in the way in which the drawings are used.
- Photogrammetry is evidenced as one of the most interesting techniques that we have today to raise the built heritage. Not only because of its accessibility, but also because of the speed and precision of its results. This fact, well demonstrated and known, is fundamentally evident since the students themselves have been, and continue to be, the ones who choose it from among all possible ones.

ACKNOWLEDGMENT
This work is developed as part of an R+D+I project entitled "The representation of time in graphic expression", with reference project-emergent-GRE18-10 and funded, in public concurrence, by the Vice-Rector's Office for Research and Transfer of Knowledge of the University of Alicante.
El proceso que permite el sugerente mundo del dibujo puede entenderse, también, como la serie de muñecas rusas en las que la finalidad de cada una de ellas termina en abrir la siguiente. Las etapas del dibujo empiezan y terminan en la definición de los límites de éste y sintetizan la serie de esfuerzos que, alrededor de lo visual, organizan las bases de datos de cada uno de los estadios o momentos. El objetivo de dibujar es poder dibujar. En este contexto podemos afirmar que la causa del dibujo es otro dibujo. Comenzando por la representación que, al habitar en el mundo nos hacemos de éste, y terminando por el soporte papel con la serie de modificaciones en tinta que representan un espacio construido, el papel del dibujo es el de servirnos de ayuda o herramienta para comprender y comunicarnos y es en este sentido en el que la mayoría de las investigaciones utilizan la grafía: como palanca para alcanzar sus objetivos.