Fourth Spanish National Forest Inventory. Field data

Once the UTM sampling points that will become plots have been selected, the next step is to transfer them to the most modern and appropriate aerial photographs available. These photographs are then passed on to the fieldwork supervisor who, after proper planning, distributes them to the team leaders. These leaders, aided by general maps of the area, available information from the previous inventory, and the photos, travel in an off-road vehicle to the vicinity of the target point. Then, on foot and using stereoscopic observation of the photo pairs, along with the access sketches from NFI3 for repeated plots, they survey the identified terrain until they reach the plot. The location of the plot’s center point, determined by a random decision without influence from topography, vegetation, operator preference, etc., is formalized by driving a tubular metal rod (rejón) into the ground, which is hidden when the team leaves. In repeated plots, this rod is already in the field and is located with the additional help of the azimuth and distance to previously measured trees from NFI3, which are re-identified, and with the use of a metal detector. If for any reason the rod cannot be found, the plot is treated as a new one. In some autonomous communities (administrative units), recent orthophotos are used instead of aerial photographs. This method has the advantage of reduced costs—modern aerial surveys of any scale can be used to generate orthophotos—and of more accurate UTM point placement on the maps. However, it lacks stereoscopic vision, which is partly offset by taking old NFI3 photograms into the field as well. Once the team leader is positioned at the center of the plot, the measurement of the parameters selected for this inventory begins, following the guidelines written in a fieldwork instruction manual developed for the purpose. A detailed description of these procedures is provided in the NFI4 Methodological Summary (available at: https://iepnb.gob.es/sites/default/files/2025-06/Manual_ifn4_v5.pdf).It’s worth noting that the sampling unit size (area to be surveyed) varies depending on the diameter of the trees (multi-radius plots).

Each plot record includes twelve information blocks:

• Identification (province, sheet, photogram, number, class, etc.)
• Classification by both photo-interpreter and field operator based on a series of parameters (land-use level, morphostructural level, species, occupancy, c ondition, etc.)
• Structure of age classes and origin of woody vegetation
• Dendrometry – the most important block – which includes polar coordinates and numbering of the trees (distinguishing between new and previously measured trees), botanical species, two diameter measurements, total height, quality, volume estimation form, phytosanitary condition, and special parameters. It also includes regeneration, woody shrub cover, and forest tree species present.
• Soil analysis – assessing stoniness, texture, organic matter content, pH, and soil type
• Risks – such as erosion or fire hazards
• Silviculture – documenting regeneration cuts and improvement treatments for canopy and soil
• Physiography – measuring and sketching the plot’s terrain
• Access and reference itinerary – described in text and graphics
• Observations – where the operator can record any relevant notes about the plot measurement
• Control data – including team leader name, start time, etc.
• Photographic documentation – describing how the photographs were taken to illustrate the landscape and visualize forest structure

As can be easily understood, collecting this large volume of intricate data requires specialized personnel, complex measuring and recording equipment, and a detailed and thorough instruction manual. While it is impossible to describe all measurement techniques here, it is worth knowing that the measurements and notes—aside from some complex drawings—are entered into a rugged, high-capacity laptop using specially designed software made specifically for this project.