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14.4. Lesson: Updating Forest Stands

Now that you have digitized the information from the old inventory maps and added the corresponding information to the forest stands, the next step is to create the inventory of the current state of the forest.

You will digitize new forest stands using an aerial photo. As with the previous lesson, you will use an aerial Color Infrared (CIR) photograph. This type of imagery, where the infrared light is recorded instead of the blue light, is widely used to study vegetated areas.

Después de digitalizar las masas forestales, añadirás información como nuevas restricciones dadas por las normas de conservación.

The goal for this lesson: To digitize a new set of forest stands from CIR aerial photographs and add information from other datasets.

14.4.1. ★☆☆ Comparing the Old Forest Stands to Current Aerial Photographs

The National Land Survey of Finland has an open data policy that allows you downloading a variety of geographical data like aerial imagery, traditional topographic maps, DEM, LiDAR data, etc. The service can be accessed in English here. The aerial image used in this exercise has been created from two orthorectified CIR images downloaded from that service (M4134F_21062012 and M4143E_21062012).

  1. Open QGIS and set the project’s CRS to ETRS89 / ETRS-TM35FIN in Project ► Properties… ► CRS

  2. Add the CIR image rautjarvi_aerial.tif to the project:

    1. Go to the exercise_data\forestry\ folder using your file manager browser

    2. Drag and drop the file rautjarvi_aerial.tif onto your project

  3. Save the QGIS project as digitizing_2012.qgs

As imagens do CIR são a partir de 2012. Você pode comparar os padrões que foram criados em 1994, com a situação de quase 20 anos mais tarde.

  1. Add the forest_stands_1994.shp layer created in the previous lesson:

    1. Go to the exercise_data\forestry\ folder using your file manager browser

    2. Drag and drop the file forest_stands_1994.shp onto your project

  2. Set the symbology for the layer so that you can see through your polygons:

    1. Right click forest_stands_1994

    2. Select Properties

    3. Go to the symbology Symbology tab

    4. Set Fill color to transparent fill

    5. Set Stroke color to purple

    6. Set Stroke width to 0.50 mm

  3. Repasa cómo las masas forestales antiguas siguen (o no) lo que puede que interpretes visualmente como un monte homogéneo.

  4. Amplía y desplázate sobre el área. Probablemente observarás que algunas de las masas forestales todavía se correspondan con la imagen pero otras no.

    This is a normal situation, as some 20 years have passed and different forest operations have been carried out (harvesting, thinning…). It is also possible that the forest stands looked homogeneous back in 1992 to the person who digitized them but as time has passed some forest has developed in different ways. It is also possible that that forest inventory priorities back then were different from those of today.

A continuación, crearás nuevas masas forestales para esa imagen sin utilizar las antiguas. Luego puedes compararlas para ver las diferencias.

14.4.2. ★☆☆ Interpreting the CIR Image

Vamos a digitalizar la misma área que cubría el viejo inventario, limitada por las calles y el lago. No tienes que digitalizar el área completa, como en el ejercicio anterior puedes empezar con un archivo vectorial que ya contiene la mayoría de las masas forestales.

  1. Remove the layer polygonLayer forest_stands_1994

  2. Add the file exercise_data\forestry\forest_stands_2012.shp to the project

  3. Set the styling of this layer so that the polygons have no fill and the borders are visible

    1. Open Properties dialog of the forest_stands_2012 layer

    2. Go to the symbology Symbology tab

    3. Set Fill color to transparent fill

    4. Set Stroke color to green

    5. Set Stroke width to 0.50 mm


You can see that the northern section of the inventory area is still missing. Your task is to digitize the missing forest stands.

Before you start, spend some time reviewing the forest stands already digitized and the corresponding forest in the image. Try to get an idea about how the stands borders are decided, it helps if you have some forestry knowledge.

Some points to consider:

  • Which forests have deciduous species (in Finland these are mostly birch forests) and which ones have conifers (in this area these are pine or spruce)? In CIR images, deciduous species usually show up as a bright red color whereas conifers show as a dark green color.

  • How old is the forest? The size of the tree crowns can be identified in the imagery.

  • How dense are the different forest stands? A forest stand where a thinning operation has recently been done would show spaces between the tree crowns and should be easy to differentiate from other forest stands around it.

  • Áreas azuladas indican terrenos áridos, calles y áreas urbanas, cultivos que todavía no han comenzado a crecer etc.

  • Don’t use zooms too close to the image when trying to identify forest stands. A scale between 1:3 000 and 1:5 000 should be enough for this imagery. See the image below (1:4000 scale):


14.4.3. ★☆☆ Try Yourself: Digitizing Forest Stands from CIR Imagery

When digitizing the forest stands, you should try to get forest areas that are as homogeneous as possible in terms of tree species, forest age, stand density… Don’t be too detailed though, or you will end up making hundreds of small forest stands - and that would not be useful at all. You should try to get stands that are meaningful in the context of forestry, not too small (at least 0.5 ha) but not too big either (no more than 3 ha).

With these points in mind, you can now digitize the missing forest stands.

  1. Set up the snapping and topology options:

    1. Go to Project ► Snapping options…

    2. Press snapping Enable Snapping and select Advanced Configuration

    3. Check the checkbox forest_stands_2012 layer

      1. Set Type to Vertex

      2. Set Tolerance to 10

      3. Set Units to pixels

      4. Check the box under Avoid Overlap

      5. Press topologicalEditing Topological editing

      6. Choose avoidIntersectionsLayers Follow Advanced Configuration

      7. Close the pop-up

  2. Select the forest_stands_2012 layer on the Layers list

  3. Click the toggleEditing Toggle Editing button to enable editing

  4. Start digitizing using the same techniques as in the previous lesson. The only difference is that you don’t have any point layer that you are snapping to. For this area you should get around 14 new forest stands. While digitizing, fill in the StandID field with numbers starting at 901.

  5. Cuando hayas acabado tu capa debería parecerse a esto:


Now you have a new set of polygons showing the different forest stands in 2012 - as interpreted from the CIR images. However, you are missing the forest inventory data. For that you will need to visit the forest and get some sample data that you will use to estimate the forest attributes for each of the forest stands. You will see how to do that in the next lesson.

You can add some extra information about conservation regulations that need to be taken into account for this area.

14.4.4. ★☆☆ Follow Along: Updating Forest Stands with Conservation Information

For the area you are working in, there are some conservation regulations that must be taken into account when doing the forest planning:

  • Se han identificado dos territorios de protección de una especie de ardilla voladora siberiana (Pteromys volans). De acuerdo con las normas, un área de 15 metros alrededor de los puntos debe dejarse intacta.

  • A riparian forest of special interest that is growing along a stream in the area must be protected. In a visit to the field, it was found that 20 meters to both sides of the stream must be protected.

You have a vector file containing the information about the squirrel locations and another containing the digitized stream running from the North area towards the lake.

  1. From the exercise_data\forestry\ folder, add the squirrel.shp and stream.shp files to the project.

  2. Use the openTable Open Attribute Table tool to view the squirrel layer

    Puedes ver que hay dos localidades definidas como ardilla voladora siberiana, y que el área a ser protegida está indicada por una distancia de 15 metros desde las localidades.

Let’s more accurately delimitate that area to protect. We will create a buffer around the point locations, using the protection distance.

  1. Abre Vector ► Geoprocessing Tools ► Buffer.

  2. Set Input layer to pointLayer squirrel

  3. Set Distance to 15 meters

  4. Set Buffered to exercise_data\forestry\squirrel_15m.shp

  5. Check checkbox Open output file afer running algorithm

  6. Clique em :guilabel:` Executar `

  7. Once the process is completed, click Close


If you zoom in to the location in the northern part of the area, you will notice that the buffer area extends over two neighbouring stands. This means that whenever a forest operation takes place in that stand, the protected location should also be taken into account.


For the protection of the squirrels locations, you are going to add a new attribute (column) to your new forest stands that will contain information about locations that have to be protected. This information will then be available whenever a forest operation is planned, and the field team will be able to mark the area that has to be left untouched before the work starts.

To join the information about the squirrels to your forest stands, you can use the Join attributes by location algorithm:

  1. Aberto: menuselection: Vetor –> Ferramentas de Gerenciamento de Dados –> Juntar atributos por localização.

  2. Set Join to features in to polygonLayer forest_stands_2012

  3. In Geometric predicate, check checkbox intersect

  4. Set By comparing to to polygonLayer squirrel_15m

  5. Set Join type as Take attributes of the first matching feature only (one-to-one)

  6. Leave unchecked Discard records which could not be joined

  7. Set Joined layer to exercise_data\forestry\stands_squirrel.shp

  8. Check checkbox Open output file afer running algorithm

  9. Clique em :guilabel:` Executar `

  10. Once the process is completed, you can Close the dialog.


Now you have a new forest stands layer, stands_squirrel.shp showing the protection information for the Siberian flying squirrel.

  1. Open the attribute table of the stands_squirrel layer

  2. Sort the table by clicking on point_pr field in the table header.


    You can see that there are some forest stands that have the information about the protection locations. The information in the forest stands data will indicate to the forest manager that there are protection considerations to be taken into account. Then he or she can get the location from the squirrel dataset, and visit the area to mark the corresponding buffer around the location so that the operators in the field can avoid disturbing the squirrels environment.

14.4.5. ★☆☆ Try Yourself: Updating Forest Stands with Distance to the Stream

Following the same approach as for the protected squirrel locations you can now update your forest stands with protection information related to the stream. A few points:

  • Remember the buffer is 20 meters around the stream

  • You want to have all the protection information in the same vector file, so use stands_squirrel.shp as the base layer

  • Name your output as forest_stands_2012_protect.shp

Once the process is completed, open the attribute table of the output layer and confirm that you have all the protection information for the riparian forest stands associated with the stream.

When you are happy with the results, save your QGIS project.

14.4.6. In Conclusion

Has visto cómo interpretar imágenes CIR para digitalizar masas forestales. Por supuesto debería llevar más práctica el refinar las masas y utilizar otra información como mapas de suelos para obtener mejores resultados, pero ahora sabes las bases para ese tipo de tarea. Y añadir información desde otros conjuntos de datos ha resultado ser una tarea bastante trivial.

14.4.7. What’s Next?

Las masas forestales que digitalizaste se utilizarán para planear operaciones forestales en el futuro, pero todavía necesitas obtener más información sobre el bosque. En la siguiente lección, verás cómo planear un conjunto de parcelas de muestreo para inventariar el área forestal que acabas de digitalizar, y obtener una estimación global de los parámetros forestales.