Repeated measurement of permanent inventory or research plots belongs to the most common and at the same time important tasks of the field data collection.
Permanent plots may be with or without marked plot center or corner, with numbered trees or with so called hidden numbering when trees are identified via their co-ordinates. During the repeated measurement it is necessary to recover plot position, position of individual trees and other entities mapped during the first measurement. Values of new attributes are recorded and might be verified in comparison with previous data.
Field-Map supports all steps and procedures which are necessary for successful re-measurement of permanent plots. Prior the fieldwork of the repeated measurement it is necessary to adjust database structure and prepare layers for the data of new measurement.
Starting point of the repeated measurement is represented by the Field-Map project filled with the data of the first measurement. Prior the fieldwork of repeated measurement it is necessary to adjust the structure of the project and pre-fill it with data where relevant.
Let’s assume simple example project with 500 m 2 circular inventory plots and layers for trees, deadwood and shrubs (polygons). Additional point layer “Marked trees” is used to record position of marked trees. The method of marked tree which is usually established in the distance about 25-30 meters and any direction from the center of the inventory plot is used to mark plot position. Such a tree is painted with reflexive paint at breast height and also at the bottom of the trunk in order to be easily identified in the field. Marked tree outside of the inventory in unpredictable direction and hardly predictable distance keeps the position of the inventory plot hidden and at the same time it simplifies the process of repeated finding of a inventory plot. One tree is usually marked per each inventory plot. The position of marked tree is mapped using local system of co-ordinates during the first plot measurement – same co-ordinate system as for the tree mapping is used.
Picture 1. Structure of the example project
Picture 2. Example project with example data of the first measurement of inventory plot
Preparation of the data for repeated measurement starts with preparation of layer for navigation. Original layer of inventory plot centers has been used during the field measurement to navigate to the plot centers.
Picture 3. Grid of plot centers (designed prior the fieldwork of the first measurement)
During the first measurement the position of marked trees has been recorded and those trees can be used for improved navigation to inventory plots. However position of marked trees has been recorded using local co-ordinates and needs to be transformed into map projection which can be combined with GPS mapping.
The centers of planned grid of inventory plots are usually prepared in map projection co-ordinates. Thus each plot has plot center co-ordinates expressed in local co-ordinate system [0,0,0] and also in map projection co-ordinates [X,Y,Z]. Both co-ordinates should be stored in “GPS reference point” layer which belongs to the standard layers of every Field-Map project. Looking to “Georeferencing” table of an inventory plot the co-ordinates can be checked or even edited.
Picture 4. Opening georeferencing table
Picture 5. Georeferencing table of a plot showing selected co-ordinate system and co-ordinates of plot center
In order to make co-ordinates available in map projection co-ordinates system it is necessary to export them into new map layer. For that the function “Combine shapefiles” can be found in Tools menu of Field-Map Project Manager. All plots except of navigation plot are combined and MarkedPoints layer is exported. New point shapefile is created.
Picture 6. Combine and export marked trees of inventory plots
In order to use newly created shapefile of marked trees in the Field-Map project for navigation it is suggested to create new point layer MarkedTrees2. In navigation plot the newly created shapefile is imported into this point layer. After that the position of marked trees become available for navigation in addition to existing grid of plot centers.
Picture 7. Co-ordinates of marked trees can be used for navigation in the field
Mapping of tree position at inventory plot represents specific task because tree co-ordinates are primarily used for tree identification. Trees have been numbered via their co-ordinates but the numbering is not obvious because the numbering is hidden. However for correct inventory it is necessary to identify every tree and also identify new ingrowth and removed tree.
During preparation of Field-Map data it is possible to copy existing layer of trees into new layer which will then be used for repeated measurement. Field-Map Project Manager provides specific function for copying layers. During copying of a tree layer it is possible to decide about the way of tree IDs’ copying.
Picture 8. Management of tree ID during copying of tree layer
For repeated measurement there are two possibilities of tree ID copying. The tree ID can be set to negative value. Thus in the field map each tree will have its respective ID visible from the beginning and those tree which will be identified in the field will change the value of ID from negative to positive.
The other option is to set all IDs to zero value. In this case all trees will have ID equal to zero and during the fieldwork new values are set to them. Since the identification of tree is primarily based on their co-ordinates the ID is secondary and there is no danger of the loss of the track of repeated measurement of individual trees. However it is also possible to store automatically old ID value into newly created attribute during copying of layer. This approach of tree numbering is more convenient for those who uses stand alone electronic caliper for tree diameter measurement. Diameters are usually measured in sequence which corresponds to the numbering of trees. Ingrowth trees are automatically included and removed trees are automatically skipped.
During the copying of a layer for repeated measurement it is possible to prepare also attributes and decide how they will be handled during repeated measurement.
Picture 9. Preparation of attributes for repeated measurement
For every attribute it is possible to decide about the way of copying data and the mode of verification of its value during fieldwork.
Value of the attribute can be transferred to the new layer of repeated measurement. It is recommended to copy only those attributes which are not assessed subjectively. If copied then the values are visible in the field form.
Optionally it is possible to use conditional value copying. If value is copied conditionally then the values are visible in the field form in italics and require approval.
On-line verification of attribute value is advanced functionality which compares new value with old one whenever the data are changed.
Off-line verification provides with similar function but it is activated on request. When requested the data of whole plot are checked and discrepancies are reported.
During resolvement of discrepancies it is also possible to change old values in the layer of previous measurement. However for that the special permission is required. Such a permission needs to be granted to each attribute individually.
Mode of verification can be set attributes during copying of the layer. However the mode can be also easily changed later.
Trees have specific position in the Field-Map project. This is because the time series is built on the level of individual trees – values of old and new measurement needs to be linked for individual trees. Similarly for deadwood.
For the other layers the situation might be different. For such a layer like polygons of brushes it might be uneasy to keep track on the level of individual entities. In such a situation it might be useful to combine copying of layer and attributes with visualization of old data in Field-Map project.
In Field-Map Project Manager it is possible to make layers visible or invisible for fieldwork. Additionally it is also possible to prevent editing of map layer and attributes. Thus it is possible to make old layer visible both in map and attribute page but without possibility of editing.
Repeated measurement repeats same or similar steps as the first measurement. The plot position needs to be identified, tree numbering recovered, and measurement and attributing is performed. However during repeated measurement one can benefit from availability of existing information. For permanent plot is not only possible but strongly required to keep track of changes on the level of individual entities, e.g. trees.
Center of existing plot can be found in three steps:
Navigation to target co-ordinates represents standard Field-Map task. During navigation it is possible to use GPS or laser+compass as well as existing digital maps, aerial photos etc.
Picture 10. Navigation to target co-ordinates (to marked tree)
Navigation usually reaches accuracy of several meters. This is sufficient to find and recognize marked tree.
Navigation from marked tree to the plot center uses same navigation procedure, this time in local co-ordinates.
Picture 11. Navigation from marked tree to plot center
Accuracy of establishment of plot center using described procedure is usually in tens of centimeters. The equipment cannot be positioned exactly at marked tree position and this is the main reason of inaccuracy.
Final adjustment of the position of plot center can be done using known position of trees. Function “Position using set of trees” has been designed in Field-Map Data Collector to get georeference from several trees which can be distinguished in the field as well as on the plot map. Practically always such a trees can be distinguished using their species, dimension or clearly identifiable position. It is recommended to use about three trees preferably in different directions.
Together with calculating position of the plot center the magnetic declination can be measured as well. This function is particularly useful when it is not clear whether the correction for magnetic declination has been used in previous measurement. Calculated of magnetic needs to be submitted to electronic compass in order to get correct co-ordinates while mapping in the plot.
Picture 12. Positioning using known tree positions
As a result of the positioning procedure the current position of the measurement device is calculated using co-ordinate system of particular plot. The position most likely will still differ from the plot center but in principle there is no reason to reach exactly plot center. It is fully satisfactory to get georeferenced and after that it is possible to establish suitable reference point and to start mapping and measurement.
The accuracy of positioning will be limited by the accuracy of measurement equipment. Usually it is better than 10-20 cm.
Re-establishment of tree numbers is necessary in order to keep time series of measurement on the level of individual trees. In case of so called hidden tree numbering the trees can be identified using tree co-ordinates and also tree description.
The tree data are prepared prior the fieldwork – tree IDs are set to negative ID value or to zero. Negative ID values are usually used in case of interactive diameter measurement either with manual caliper or with electronic radio caliper which submit diameter to the computer tree-by-tree.
Zero tree IDs are usually used when tree diameters are measured with off-line electronic caliper. Tree diameters are stored in calliper’s memory and downloaded to the field computer when whole plot is measured. Therefore it is necessary to maintain same sequence of tree IDs both in caliper and computer. Trees are numbered sequentially.
Picture 13. Negative tree IDs prepared for repeated measurement
Picture 14. Zero tree IDs prepared for repeated measurement
Recovering of tree numbers uses same procedure as mapping of trees. Get equipment positioned, start tree mapping and shoot with laser rangefinder to a tree.
Picture 15. Mapping of tree position
Upon shooting to the tree Field-Map map shows trace of the laser beam (from equipment position to the target) and circle of search tolerance. Due to the accuracy limits of measurement equipment it is hardly possible to reach exactly same co-ordinates of target trees when measuring to the target position on the surface of trunk. In practice there will be always some difference in millimeters or centimeters. Therefore the direct identification using tree co-ordinates is not possible. Instead of that Field-Map uses search tolerance for searching of neighbor trees which are still “available” for tree identification. The list of such trees is presented in right side of the Field-Map screen. Trees are ordered by distance to current target position and completed with description (species, DBH, height). Descriptive information helps to distinguish trees in the field.
Sear tolerance is usually adjusted in relation to the density of forest stand. More dense is the forest the less search tolerance needs to be used.
When the tree is identified then it is accepted and its ID changes value from negative to positive. After that the whole procedure is repeated for other tree.
Trees which have been already used in identification process appear in the list of closest trees by strikethrough font and cannot be used any more. However if for some reason emerges that tree identification was wrong the identification can be reset. In such a case tree ID again becomes negative and needs further identification.
In practice there is often not necessary to shoot with laser rangefinder to every tree. Especially in less dense forest stands it is possible to identify tree visually and click to the tree position just by pen. The rest of the procedure remains the same. In case of clear situation at the plot it is possible to identify all trees in one step. For that there is “Accept all trees” function under tools menu. Another possibility is to “Accept selected trees”. Subset of trees is selected using pen and after that identified in one step.
If new (ingrowth) tree is mapped then it is necessary to give him new ID rather than identifying it with one of the original trees. In such a case the button “New tree ID” is used. The value of new ID can follow existing series of tree IDs or can use specific base in order to distinguish new trees. If for instance base number of 100 is used then new trees will start with ID=101 etc.
When tree identification of whole is finished then it is possible to check unassigned trees. If everything is correct then unassigned are those trees which have been removed. Those trees most likely need to be removed from the database as well. “Delete unassigned trees” function makes this operation easy.
If zero ID method is used then the trees are numbered sequentially with IDs which differ from previous measurement. The link however is kept using tree co-ordinates. New trees (ingrowth) is mapped during tree identification, new trees are numbered in the same sequence. At the end of identification procedure some trees with zero ID may remain on the map. Those are the trees which have been removed from the forest and needs to be removed from the database.
In some cases it may happen that tree position has been mapped wrong in previous measurement. In such a case it is possible to use “Move point” function and correct the position. Description of any change done to the old layer is automatically stored in OldChangesLog table which is part of Field-Map project.
In order to increase productivity of fieldwork and to improve quality of data the attributes might be prepared several ways (see Adjustment of verification parameters for attributes chapter).
The values of attribute can be copied (e.g. tree species), copied conditionally (e.g. DBH) or made ready for verification.
Conditional copying of attribute value means that the value is visible in field form. It is presented using italic font and requires replacement by new value or confirmation.
Picture 16. Field form with pre-prepared attributes prior fieldwork
On-line verification is run automatically whenever the attribute value has changed. New value is compared with the old one and if the difference is found then the confirmation dialog appears. User has three options for choice:
In order to make this type of validation available the option of “Old change” needs to be activated during data preparation.
If for some reason the old value of attribute is missing then the old value is automatically filled by the new value.
Picture 17. Confirmation dialog for attribute verification
Somewhat more complex is the situation when conditional lookup list is verified. Available items of conditional lookup list depend on master attribute and therefore in some cases the value of old and new attributes cannot be changed – the only option is to accept both values as they are.
Picture 18. Confirmation dialog for attribute verification – conditional lookup list with different
items as a result of different value of master attribute
Values of conditional lookup list attributes are changed automatically upon the change of master attribute. If for instance conditional lookup list has only one item for particular master attribute then it is filled automatically. If there are more options then the value remains empty.
The behavior of verification can be adjusted using settings. It is possible to switch the on-line verification off and it is possible to switch on/off the appearance of confirmation dialogs for conditional lookup list attributes. If it is switched on then the sequence of confirmation dialogs appear in reaction to the change of master attribute. If it is switched off then the changes of conditional attributes are made as well but the dialog is not shown.
Picture 19. Settings – options for attribute verification
Off-line verification of attribute values means that the verification is done on user’s demand. When the verification procedure is run then all data of current plot are checked according to predefined verification scheme. The results of verification are presented in separate dialog for further resolving of conflicts.
Picture 20. Off-line verification of attribute values
Off-line procedure of attribute verification processes data of current plot and prepares output window with the list of conflicts.
Picture 21. Output window of off-line verification procedure
The list of conflicts between old and new values of attribute presents full identification of records (layer name, attribute name and ID) and also current values of attribute.
Attributes of conditional lookup lists are marked by ©. Below the list the identification of master attribute can be found.
Missing (null) values are marked as
“Resolve” button or double click to the selected item of the list starts procedure of attribute verification. This process is exactly the same as the process used for on-line verification. Thus it is possible to resolve conflicts for all attributes step-by-step.