重要

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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.

将林分数字化后，您将添加信息，例如资源保护条例给出的新制约条款。

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

该CIR图像来自2012年，可以将1994年创建的态势图与近20年后的情况进行比较。

  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. 查看旧林分是如何遵循 (或不遵循) 您可能在视觉上解释为同质森林的内容。

  4. 围绕该地区缩放和平移。您可能会注意到，一些旧林分可能仍与图像对应，但其他林分则不然。

    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.

接下来，您将为该图像创建新的林分，而不是使用旧林分。稍后可以比较它们的异同。

14.4.2. ★☆☆ Interpreting the CIR Image

让我们将旧清单覆盖的同一区域数字化，该区域受到道路和湖泊的限制。您不必将整个区域数字化，就像在上一个练习一样，您可以从已经包含大部分林分的矢量文件开始。

  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

    ../../../_images/stands_2012_1.png

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.

  • 青色区域表示荒芜地带、道路和城市区域、尚未开始生长的农作物等。

  • 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):

../../../_images/zoom_to_CIR_1-4000.png

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

      ../../../_images/snapping_2012.png

  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. 完成后，您的图层应如下所示：

    ../../../_images/new_stands_ready.png

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:

  • 已经确定了两个西伯利亚鼯鼠 (Pteromys volans) 保护物种的位置。根据规定，位置周围15米范围内不得触碰。

  • 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

    您可以看到，有两个位置被定义为西伯利亚鼯鼠栖息地，要保护的区域由距这些位置15米的距离表示。

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

  1. 打开 矢量 ► 地理处理工具 ► 缓冲区 。

  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. 单击 运行

  7. Once the process is completed, click Close

    ../../../_images/squirrel_15m.png

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.

../../../_images/north_squirrel_buffer.png

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. 打开 矢量 ► 数据管理工具 ► 按位置连接属性 。

  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. 单击 运行

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

    ../../../_images/joined_squirrel_buffer.png

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.

    ../../../_images/stands_squirrel_table.png

    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

您已经了解了如何领会CIR图像并将林分数字化。当然，制作更准确的林分需要一些实际操作，通常使用其他信息 (如土壤分布图) 会得到更好的结果。现在您已具备了完成此类任务的基础，而从其他数据集添加信息则是一项非常琐碎的任务。

14.4.7. What's Next?

您数字化的林分将用于规划未来的林业作业，但仍需获取有关森林的更多信息。在下一课，您将了解如何设计一组采样图表，以清查刚刚数字化的森林面积，并获得森林参数的总估算。