Introduction
For the
navigation subject, the preparation before going to the field was already
covered, and in the last week, the first field activity was done using the
traditional method: map and compass. Now it’s time to deal with the use of a
GPS unit for navigation. However, the map produced before won’t be available at
this point. In the next exercise, though, not only the GPS unit will be used,
but also an improved map for navigation. Finally, a comparison of all the
methods along these weeks will be done.
This week’s
activity, the navigation with the GPS unit, occurred on March 11th,
2013 and the main goal was to analyze how well this technique works, comparing
to the traditional way with a compass and map.
Methods
Considering the
depth of the snow in the last activity and the permanence of the weather
conditions, if not worse than before, it was necessary to improve the clothing
preparation. Then, water-proof and a higher number of layers were used.
The only
equipment used in this exercise is a Garmin Etrex GPS Unit (Figure 1),
supported by a table containing the UTM coordinates for each point. After a
quick overview about the basic use of the unit, each group was directed to the
corresponding course.
Figure
1 – Garmin Extrex GPS Unit
This report
refers to the procedures taken by the Group 1, who navigate by the course
number 2, from the first point to the sixth point. The track log mode was
turned on without any change in the settings. For the first points in the
course, a method was applied to find them; and then another for the further
points. The first consists in fixing one of the coordinates and, after that,
fixing the other coordinate. This way takes a longer time, but would guarantee
to arrive in the correct place. Then, the X coordinate was taken in
consideration, and after being in the correct place in the X axis, the same was
done to the Y coordinate. By looking at the changes in the coordinates on the
unit screen, it was possible to guarantee that the direction was correct. The
internal compass of the unit was also used to be certain of the direction.
However, after
two points, it was noticed that a more convenient and effective way could be
used to navigate from one point to another. The GPS contains a tool called “Where
to?”; the coordinates for the target point are input there, and the unit will
automatically show the direction on the compass and the distance to there. With
this way, a lot of time was saved, and it wasn’t necessary to keep the eye on
the GPS all the times, allowing to be aware of the environment around more
carefully. Also, the direction and distance were automatically update as the
group moved, so there were no worries about the lack of precision in case the
track was missed because of some obstacle – such as elevations, dense
vegetation or restricted areas. The precision was never perfect since the area
consists in woods, but the margin of error, based on the PDOP, could be
constantly monitored and kept in an acceptable level.
After passing
through all the points, the tracklog was turned off and later downloaded in the
computer. For that, it was necessary to examine the DNR Garmin software,
connecting it to the GPS unit by an USB cable and acquiring the data stored in
it. The data can be exported as a shapefile based on points or lines. The first
choice was line, but later on it was known that the ideal type would be points.
Then, Arc Tool Box was used, and the command “Feature Vertices to points”
enabled the conversion from lines to points.
All the students
were supposed to follow the same process and all the data would be available for
the class. The files were located on a protected folder, reason why it was preferable
to create a new geodatabase in the personal folder – where editing is allowed –
and import the data (Figure 2). Besides the geodatabase, dataset were created
to maintain organization and guarantee the coordinate system uniformity. In
this case, the Eau Claire County System was used: since the tracklogs would be
used only for presentation purposes, it was no longer necessary to use the UTM
coordinates. In this case, the coordinate system covering the smallest area
will have the minimum distortion and that’s why the county coordinate system
was chosen.
Figure
2 – Geodatabase
However, as it happened before,
some track logs were available as lines, so it was again necessary to run the “Feature
vertices to points” command in Arc Tool Box. After having all the data prepared,
three maps were elaborated: an individual map containing my own path in the
activity; a group map referring to the tracks from each of the group components;
and, for last, a map containing the paths taken for the whole class components.
The course points were also available,
so they were added to all the maps and symbolized accordingly to the course they
belong to. Also, it would be interesting to have an idea of the shortest path
that could be taken between the points. For that, it’s necessary to create
lines between the points. The command “Points to Line” in the Database
Management in the Arc Tool Box was used to have this result. However, if it was
simply run only inputting the source and output, there would be a connection line
between all the points. That wouldn’t represent the idea of three different
courses, as it’s needed. Then, the “Line Field” was use to indicate which field
in the attribute table would differentiate the lines created.
Figure
3 – Use of Point to Line tool
Then, it was
possible to symbolize both lines and course points accordingly with their
course. Labels were also used to assess the readability of the maps. Different
levels of emphasis were used depending on the map being made and the amount of
data associated. Transparency and lighter colors were used when these features
were not the main point of the map.
For the map with
the track logs of all the students, they were separated in the corresponding groups
and each group would have the same color to help the interpretation. However,
when dealing with the group map, the three track logs were symbolized in
different colors to analyze the differences and similarities between them. In
this case, since the other courses were not part of the analysis, a higher
scale focusing only in the second course was use. The same scale was used for
the individual map, as well as a higher tone for the points and lines of the
course, so a comparison would be easily done.
The maps were
essential to support the analysis of accuracy and precision of different
collections, as well as to notice different behavior taken in the path,
depending on the obstacles found. The use of the satellite image was important
to recognize different types of vegetation and its effects on the paths taken.
Discussion
In terms of accuracy,
it’s interesting to examine the individual map (Figure 4). In the fourth course
point, the GPS acquire some points close or even on the highway, where the group
clearly didn’t go to. The same problem doesn’t occur in most of the other
places and although the area was vegetated, it wasn’t much different for the
others.
Figure
4 – Individual Track logs map
Thus, the most
reasonable explanation for this lack of accuracy at this point is the fact that
in the fourth course point, the group stopped for a moment to rest and set the
GPS to the next point (Figure 5). The longer time at this point can be noticed
by the high amount of points taken there. When dealing with a path, collection
done by a GPS have the accuracy compromised when the GPS unit stays for a
longer time in the same position. It’s different when collecting a point
feature class, where the unit collects a number of points, ignore the outliers
and calculate the average of the others.
Figure
5 – Quick stop to rest and set the GPS to the next point.
It’s also
possible to notice that after the sixth point, the collection soon was stopped.
That was not intentional, but the battery was low and after the sixth point,
there was no much need of looking at the GPS so frequently, then it turned off
automatically and this was only noticed later.
As well as in
the last exercise – with the map and compass – the snow depth in this activity
was really high (Figure 6), compromising how fast the group could move between
points. Then, a tactic used was to avoid hills and dense vegetation areas –
that would compromise even more. So, the natural trails were used as much as it
was possible. This involves a higher distance, but would be more effective.
Figure
6 – Snow depth reaching Andrew’s knees.
This behavior
taken in the paths can be notice both in the individual map, but also in the
group map (Figure 7): all the track logs follow the contour lines, showing that
there was not a high change of elevation by avoiding hills. Between the fourth
and fifth point, the track taken was also longer than it could be, because an
area closed by fences was being avoided. Another feature avoided was the dense
vegetation at east, the group only got inside tit when it was really necessary
to get to the point. To walk through it was complicated, so the trail was preferred.
Figure
7 – Group Track logs Map
As said before,
the settings for the track log in the GPS units were not changed before going
to the field. This is very clear when comparing the three tracks in the group
map. There’s much more points in Kent’s track log, making it even look like a
line, while the other two track logs have a smaller amount of points collected.
This is part of the settings for the track log, you can set the time interval
for the data collection: the lower the time interval, the more points you will
get. It’s necessary to find the balance between having a good amount of data, but
without compromising the file storage in the GPS.
For last, a map
covering the tracks taken by all the students in this class (Figure 8) can give
a general idea of the activity. There were six groups divided in three courses,
odd number groups would go to the points in ascending order, while even number
groups would do it in descending order. Groups 1-2 were supposed to be in the
course two, groups 3-4 in the course three and groups 5-6 in the course one. More
or less, all the groups were able to navigate over the corresponding courses
and the paths taken were similar. Thus, it’s possible to affirm that the groups
had kind of the same idea and completed the task successfully.
Figure
8 – Class track logs map.
Conclusion
The activity for
this week proceeded much smoother than in the week before. The points were
easier to be found, the navigation itself took less than two hours and all the
points were covered. However, that doesn’t necessarily mean that the GPS
navigation is better than with the map and compass. Specifically for this
group, because mistakes were made, the navigation with map and compass was
complicated. However, if the appropriate steps were taken, trusting in the compass,
a different scenario would be in comparison.
Still, the step
count and the need to stop every once in a while to maintain the compass
direction delay the process; while with the GPS, the path is automatically
corrected in case it goes out of the direction. Then, in this matter, there’s
no doubt that the GPS navigation is more effective than with the map and
compass.
In the other
hand, the precision can be an issue when dealing with the GPS. As mentioned,
when the unit is standing in the same place, the accuracy is compromised. It
was essential to have three units per group: sometimes a single unit had a high
error, putting in doubt if the flag found belonged to the appropriate course.
In these occasions, to check other GPS units was useful to guarantee the right
placement. The problem with the GPS lack of accuracy is that it’s not possible
to know which element has a problem: direction or distance. By using the
compass and pace count, the analysis of which one might be dubious.
However, these
errors didn’t compromise the navigation, which happened very well, even being
in a dense vegetated area – where the PDOP tend to increase. Therefore, it’s
not a surprise that the traditional mode was taken over by the technology of
the GPS. Especially when dealing with areas not affected so much by multipath
effects and other types of errors, there’s no doubt that the GPS navigation is
more effective and appropriate for the fast paced routine most of companies and
governments have. It doesn’t mean, though, that the traditional technique
should be neglected. Although it’s not the preferable way to acquire data or
navigate, it should always be known by the geography professionals, so they
know how to deal with their tasks if the technologies fail on them.
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