Having defined the research areas and having selected the control points for the direct analysis, the control network was verified in the field. This was a very important stage of the procedure, necessary for its correct analysis. The field inspection included the analysis of information about each point according to the previously defined 9 criteria:
1. Existence of the control point in the ZGiK database,
2. Accessibility of the point for inspection,
3. Occurrence of the point in the field,
4. Existence of topographic description of the point,
5. Consistency of the point location with the graphic part of the topographic description,
6. Consistency of the B, L coordinates with the actual location of the point in the field,
7. Possibility of performing surveys on the point,
8. The point has been built-up,
9. The point has been destroyed.
These criteria were analysed for each of the points in an appropriate order, resulting from the planned procedure (Fig. 7). The result of the conducted field inspection is the collected information presented in fragments in the exemplary Table 2.
Table 2. Criteria for analysis of control network in the field.
Source: Authors’ original contribution
153 out of 799 points retrieved from ZGiK were analysed. They were located in separate areas, according to the adopted criteria. The summary of the number of the analysed points is contained in Table 3 and in the form of a pie chart (Figures 8 and 9).
Table 3. Results of field inspection conducted for selected database of points.
Source: Authors’ original contribution
Zone type
|
NUMBER OF ALL POINTS FOR SELECTED AREAS RETRIEVED FROM ZGiK DATABASE
|
NUMBER OF POINTS ADOPTED FOR ANALYSIS RETRIEVED FROM ZGiK DATABASE
|
NUMBER OF POINTS FOUND IN THE FIELD
|
Ratio of points found to analysed
[%]
|
City Centre
|
44
|
44
|
31
|
70.5
|
City Centre Outskirts
|
53
|
53
|
31
|
58.5
|
Towns
|
24
|
23
|
10
|
43.5
|
Rural Areas
|
83
|
33
|
13
|
39.4
|
TOTAL
|
204
|
153
|
85
|
55.6
|
When summarising the obtained results regarding the occurrence of points in the field in individual point location zones (City Centre, City Centre Outskirts, Towns and Rural Areas), the following conclusions can be drawn (Fig. 9):
- the greatest number of points were found in the City Centre zone,
- the greatest number of non-existent points were in the Town zone,
- the greatest number of inaccessible (hard-to-reach) points for analysis were located in the Rural Area zone.
Comparing the area of the City zone divided into the City Centre CC and the City Centre Outskirts CCO, a large discrepancy in the number of inaccessible points should be noted. In the case of the City Centre Outskirts, this number reaches 13.2% and is much higher than the number of points inaccessible for analysis in the City Centre zone, where it is approximately 4.5%. This is due to the fact that in the City Centre CC, most of the points are located on the main streets on the front walls of tenement houses, thanks to which they are generally accessible. Moving towards the City Centre Outskirts zone, more and more points are located on buildings of single-family houses, which are often fenced. The same tendency can be noticed in other point location zones, such as Towns and Rural Areas, where most of the properties belong to private owners. Fences significantly limit access to control points, which in the case of vertical control networks must frequently be placed on buildings, i.e. on private land. These statistics would probably look different for the horizontal control network, the location of which is usually chosen outside the private area in order to increase its accessibility in the field.
Attention should be paid to the disproportions that occurred in the Towns zone. Taking into account the entire database for this area, an even distribution between the found and non-existent points is noticeable (Fig. 9). There were 5 non-existent points in the Towns zone database (1 was destroyed, 4 were not found), which were also marked in the Geoportal as “destroyed points”. They were all located at the main roads. It can be assumed that this condition is related to road modernisation, during which points can be easily damaged or destroyed.
However, there are also points that were not found during the field inspection, and according to the Geoportal they are still in good condition! The largest number of such cases was recorded in Skawina (Towns point location zone). They are presented in Figure 10. Such points from the entire Towns zone database are contained in Table 4.
Table 4. Information on points presented in Figure 9 not updated in Geoportal, collected in the field. Source: Authors’ original contribution
Point number
|
Latitude
|
Longitude
|
Is the point accessible for inspection?
|
Does the point exist in the field?
|
Is there topographic description of the point?
|
Address
|
Does the point exist in Geoportal?
|
Condition of point according to Geoportal
|
17330012
|
49° 50' 02,1"
|
19° 56' 11.0"
|
YES
|
NO
|
YES
|
Myślenice, Królowej Jadwigi F. P. W. in the church square from Kilińskiego str. side
|
YES
|
good
|
17310111
|
49° 58' 21,0"
|
19° 48' 26.1"
|
YES
|
NO
|
YES
|
Skawina, 11, Piłsudskiego str.
|
YES
|
good
|
17310117
|
49° 58' 40,7"
|
19° 49' 12.3"
|
YES
|
NO
|
YES
|
Skawina, 13, Tyniecka str.
|
YES
|
good
|
17310122
|
49° 58' 38,2"
|
19° 49' 58.0"
|
YES
|
NO
|
YES
|
Skawina, 19 Krakowska str.
|
YES
|
good
|
17310212
|
49° 57' 55,0"
|
19° 49' 21.8"
|
YES
|
NO
|
YES
|
Skawina, 2, Spacerowa str.
|
YES
|
good
|
17310200
|
49° 57' 28,2"
|
19° 49' 35.4"
|
YES
|
NO
|
YES
|
Skawina, extension of Dębca str.
|
YES
|
good
|
It is also worth mentioning that in addition to the point analysis conducted by the authors there was also a parallel process of control geodetic network review conducted at the request of GUGIK [URL 14]. The results of this review are now accessible at [GEOPORTAL 2, Mapy, 2020]. But despite the lack of access to the sample point 17330012 presented in figure 11 its “Stabilization status” is still presented as “good”. Such information may lead to problems for potential users.
Despite the fact that the analysis covered Towns, which are equally intensively developing, significant differences can be noticed between them in the maintenance of the control points. In Myślenice, 57.1% of the points were found, which was the majority of accessible points in this area. However, in Skawina the statistics revealed the opposite situation, namely 55.6% of the points were non-existent. This probably resulted from the unfavourable location of the control points in the area, which made them inaccessible. The last group is Rural Areas point location zone, where most of the points are located on single-family houses. The effect is that there is the greatest number of inaccessible points compared to other types of areas. The analysis demonstrated that only 33.3% of the points found could be used for surveys.
The next stage of the analysis concerned only those points that were found in the field. The diagram presented in Figure 7 illustrates the analysis of the remaining features of the points concerning:
- their fitness to perform surveys,
- their damage,
- consistency of their location with the presentation in the graphic part of topographic descriptions,
- consistency of their location with the location indicated by the B, L coordinates.
Considering the condition of the points found for the entire test database consisting of 153 points, it could be noticed that the location of all of them was consistent with the graphic part of topographic descriptions (Fig. 12). It was found that the surveys were not possible at 11 points, 10 of which were built-up as a result of renovation works performed, and 1 was destroyed. Such a small number of destroyed points is due to the fact that they are made of durable material and, as far as possible, protected by the owners of private facilities.
The conclusion of the conducted analysis of the database of points of the basic second-order vertical control network is that majority of points that are in good condition are located in the City Centre zone. Nearly 50% of the control points that exist in the ZGiK database were not found in the Rural Areas and in the Towns zones. Out of 85 of all found points, 10 were built-up and one was destroyed, i.e. it was not possible to perform surveys on these points. Most commonly, it is building owners’ fault, who unknowingly built up benchmarks, preventing their proper use.
When analysing the reference database, it should be noted that a large part of the points, as much as 36%, have imprecise B and L coordinates, which causes discrepancy between the position of the points displayed in Google Earth (source: ZGiK coordinates, 2019) and the actual status in the field. The possible reasons for the existence of incorrect point coordinates were described in [Banasik, Ligas, Kudrys, Skorupa, Bujakowski 2012]. This fact is summarised in the bar charts below (Fig. 14), which indicate a necessity to propose a tool to improve the current state.
The greatest number of identified discrepancies occurs for the CC point location zone. It may be due to dense development and high buildings, hindering the accuracy of the surveys (Fig. 15).
For the CCO and T zones, i.e. less built-up areas, the location of most of the points corresponds to the data contained in Google Earth. However, for the CC and RA zones, this discrepancy exceeds 50% (Fig. 15). The factors identified in the presented analysis, resulting in the inability to use control points correctly, confirm the necessity to introduce tools that enable their updating on an ongoing basis. The status of the test point database demonstrates the constant dynamics of changes that are not recorded in real time. Therefore, the regular transfer of information about the occurring changes by direct users of the control network seems to be the best form of maintaining a high quality database. Thus, the solution would be a mobile application on a mobile device, such as a mobile phone, accessible directly in the field. An exemplary data update using the developed Metrica application is presented in the figure 16.
3.1. Examples of identified irregularities in the control network
This part of the research paper presents selected examples relating to the determined criteria of the control network analysis. For specific situations 1-9 that were encountered, the appropriate A-G procedures may be applied to update data using the mobile application. The aim of the A-G procedures is to collect consistent information about the control network and then its correct transfer among surveyors, which will result in the update of the control network on an ongoing basis, together with the presentation of the current status in real time. The selected examples relate to the most common criteria in the analysed control network database, where data update is required.
No access to visual inspection of the point - criterion 2
The analysis of the point database was possible under the basic conditions: the point was accessible for inspection - criterion 2, and the point existed in the field - criterion 3. The given example demonstrates a point located on an unused building, in a fenced area (Fig. 17). Verification of information about the point and the possibility of its surveying are hindered, because entering the area in the absence of the owner is not allowed.
Providing comments on hard-to-reach points makes it possible to plan surveys in the field more precisely, which greatly affects the economy of work - procedure A in the diagram (Fig. 7).
Comparison of the point location: provided by ZGiK with the actual status in the field - criterion 6
When verifying the information on the vertical control points, the source material included the topographic descriptions of points - criterion 4, as well as the B, L coordinates retrieved from ZGiK - criterion 6 (Fig. 18). These coordinates are necessary to determine the location of points in the field. The Google Earth application was used to present the location of the points. The function which can be used by a surveyor in the control network analysis, is based on the location of geodetic points. They are added by importing data from a file or by manually entering the coordinates.
When comparing the position of the points displayed in Google Earth with the actual status in the field, significant discrepancies in the location of 31 points out of 153 were noticed. The figures in Table 5 illustrate several situations of location discrepancies. It is important to note that this issue occurs in each of the selected point location zones. The presentation of the actual location of the point is marked in red, while its incorrect indication based on the B, L coordinates retrieved from ZGiK is marked in yellow. The encountered discrepancies reach up to 100 m.
This type of error is very severe. There are many points of the vertical control network that present the actual location of the point for which there are no topographic descriptions. Only the address is given, which may frequently be insufficient to correctly determine the position of the benchmark. Therefore, it is especially important to enter the location coordinates correctly. The solution for the situation presented in the example for criterion 6 (Table 6) are surveys of navigation coordinates using a mobile phone and entering the measurement result into the mobile application - procedure F from the diagram (Fig. 7). This activity will help other people using the point to locate it correctly.
Table 6.Comparison of point coordinates retrieved from ZGiK with surveys using mobile device. Source: Authors’ original contribution
Point number
|
Coordinate source
|
B
|
L
|
16330023
|
ZGiK
|
50° 05' 22,7"
|
19° 57' 28,4"
|
Survey of navigational coordinates
|
50° 05' 21"
|
19° 57' 28"
|
∆
|
1.7"
|
0.4"
|
The captured values of the B, L coordinates after the survey should be entered using the mobile application for updating the information of geodetic network points as appropriate for the correct location of the control point.
Possibility to perform surveys on a point against information that the survey marker is destroyed - criterion 7
By analysing the control network in accordance with criterion 7, it was found that 4 points had incorrect information on the topographic description, informing about their destruction (Fig. 19). The condition of the points was verified in the field and an exemplary topographic description is presented in the photo (Fig. 19). The survey marker is in good condition, it is still possible to measure it, it is located in the place indicated by the graphic part of the topographic description.
In such a situation as presented in the example for criterion 7, erroneous information must be updated. It should be done by adding a photo of the point and a new topographic description, measuring new B, L coordinates and providing information about the possibility of performing surveys on the point - procedure D in the diagram (Fig. 7).
Built-up point - criterion 8
Nowadays, we often deal with survey markers in theoretically very good technical condition which, after being built-up with insulation, become useless for levelling surveys. The reason lies in the ignorance of the owners who, wanting to protect them, are not aware of how surveyors use these points during surveys (Fig. 20). Similar situations were encountered 10 times during the control network analysis.
In the cases presented in the example for criterion 8, a surveyor using the application should post a photo and assign the point status as “not fit for surveys”. He or she may also provide information about the inability to perform surveys on the point, stating a specific reason for such a situation in the comments. In the application, the proposed scheme of conduct is point E from the scheme (Fig. 7).
Lack of information about point destruction - criterion 9
Few destroyed points were identified during the field verification which still function as useful for surveys. In the example illustrated in the photograph (Fig. 21), it can be seen that the survey marker has been destroyed. It is therefore not possible to perform a survey on this point.
In the case presented for criterion 9, the surveyor using the application should post a photo showing the actual point, give it the status of the point “not fit for surveys/destroyed” and include this information in the topographic description. In the comments to the point, it is possible to provide detailed information on the current situation. In the application, the proposed scheme of conduct is point E from the scheme (Fig. 7).