Intro to Mosaicking

Introduction

This short report refers to a specific procedure taking part on the Balloon Mapping Project, where aerial images from the University of Wisconsin - Eau Claire (UWEC) campus were taken with a simple camera elevated by a helium balloon. Individual images are interesting but to see all of the images put together can give a much better view of the location.

It’s possible to do that in a number of ways, such as using MapKnitter, ERDAS Imagine and Arc Map. In a first hands-on the procedures, the software used didn’t matter much, but in this second activity, Arc Map was focused. The class worked together dividing tasks equally over the campus, and the result should be an update imagery for UWEC campus.

Methods

The very first step is to select the best images to be used. Since the camera can be tilt and not necessarily in the perfect focus, an analysis of the more vertical and clear is essential to have better results (Figure 1). Also, the images need to have an overlap between each other of at least 60% and cover the entire campus.

Figure 1 – Oblique Vs. Vertical Images

Although intuitively the next step seems to be just to put everything together and match, when you take simple pictures with a camera, they are not georreferenced, so a mosaic tool wouldn’t work at this point. Therefore, the Georreferencing tool in Arc Map is used to give the right coordinates to the points over the image (Figure 2). The accuracy is improved as much control points are added, so a minimum of nine points per image was established.

Figure 2 – Georreferencing

After the images are correctly georreferenced, it’s time to put them together. In this step, an important point is to figure out the order of the images: the best images should be on the top, and the worse on the bottom. Also, it’s necessary to try different ways to avoid the string between the balloon and the ground (Figure 3), working with images taken in different angles. Then, the Mosaic to New Raster tool is used to produce the mosaicked image.

Figure 3 – Presence of string in the images.

Since this process is time demanding, the 18 students in this class divided the tasks to increase the efficiency and quality of the results: if each students have less images to be georreferenced, it’s possible to do it more carefully and with a higher precision. Therefore, the campus was divided into six areas, where groups of three would work in (Figure 4). For our group, five images were georreferenced for each one, and then mosaicked.

Figure 4 – Campus divided in 6 evenly polygons.

Discussion

Precise ground points were collected to improve accuracy when georreferencing, however, all of them are located in lower campus, while the section taken by the group was in the upper campus. Therefore, both imagery and the buildings feature class could be used as a reference. The buildings feature class didn’t match with the imagery though, probably because of a distortion in the imagery (Figure 5).

Figure 5 – Buildings feature class Vs. Imagery

Although the best would be to stick with the most precise – the buildings – there were some areas where there were not enough building corners (Figure 6), so the imagery would have to be used. Using two sources as a reference that doesn’t match each other was not a good idea, so only the imagery was used.

Figure 6 – Area lacking in building to use as ground control points.

When the five images chosen by each component of the group were ready, they would be grouped as a layer to ease the use of transparency and maintain organization (Figure 7). In that way, it was possible to analyze both imagery and pictures at the same time.

Figure 7 – Transparency Settings

The control points were focused in the area where each component was responsible for, since the same area – not completely accurate – would be overlapped by a better georreferenced image. After the georreferencing were completed by all the components, the mosaick to new raster tool could be directly used from the JPG, without the need of exporting it as a raster dataset.

Conclusion

The georreferencing activity can be time-demanding and require attention to detail, which can make it a really hard procedure to be done for the entire campus. However, the division between all the classmates allowed this activity to be efficient and productive.

It was also an interesting activity since the class needed to use their own resources and talk to each other to learn how to use the tools and the theory behind each procedure. More in this section will be covered in the final report for the Balloon Mapping Activity, where each step for the entire project will be explained.

Final Field Navigation

Introduction

Navigation is basically the mains one uses to locate from one place to another. That can happen in many ways and it’s a common task from the day-a-day: when you use a car GPS to give you the routes from home to work or in a trip, when you give the directions to someone at the streets. It’s more crucial when dealing with transportation by air or water, where small mistakes can have big consequences. It’s also one of the reasons why geography had extreme development when the European countries started to explore the “New World”, centuries ago.

The typical navigation requires the knowledge of the points you want to get, in a given coordinate system. The way to get to this point can be divided in two main ways: the traditional, counting on a compass and pace count; and with a GPS unit. In both ways, a map with the main features can help the identification of the main elements in the surrounding area.  In this project, all these mains will be explored and tested.

Firstly, the map is produced as a way to be a reference support to the reader, who can interpret it and potentiate the knowledge about the area one is. Therefore, map production includes a clever selection of the appropriate elements for reference, as well as dealing with different levels of emphasis in each of them. Then, the use of a compass can provide the true direction where one needs to go, while the pace count will provide de distance walked. Alternatively, the GPS unit can provide both elements automatically updated as you walk. The comparison of all the methods and the understanding of the issues related to all the processes will provide a productive analysis of navigation methods.

Study Area

The navigation exercises were done at The Priory (Figure 1), a property with 112 mostly wooded acres and three building complexes, having approximately 80 thousand square feet. It was bought in 2011 by the University of Wisconsin - Eau Claire, it is located three miles south of it, in the town of Washington. (UWEC, 2013).

Figure 1 – Aerial Image of The Priory, by Google.

Besides the constructed area, where a recreational and educational center is located, a large open area was available for the exercises, containing three courses. The courses intersect each other, containing five points each where an orange flag is located (Figure 2). The activities were done during the month of March, so the area was entirely covered with snow in a high depth. Also because of the season, the extremely dense woods didn’t have any leaves, so the mobility through the areas was challenging (Figure 3). However, some trails were available, although hard recognizable due the amount of snow. Since part of the exercise includes the use of paintball equipment, some areas were considered restricted, where it was totally prohibited to engage in any type of shooting.

Figure 2 – Courses at The Priory

Figure 3 – Weather Conditions at The Priory

Methods

The complete project totalized four weeks: preparation and planning in the first week; navigation on site with compass and map in the second week; navigation on site with a GPS unit, but no map; and for last, navigation with GPS unit and a map.

During the first week, the area was analyzed by gathering data related to elevation, dealing with the digital elevation model – obtained by USGS – as well as a two feet contour line – surveyed by the University of Wisconsin – Eau Claire, at the moment the purchase of the property was done. Other elements for the area, such as buildings and vegetation density could be examined by the imagery obtained by the Wisconsin Regional Orthophotography Consortium (WROC) in 2010. The map production should be then made thinking of the reference features that could support the navigation on site.

Also for the first week, the pace count was made to provide the distance element for the traditional navigation. Using a laser device, a 100 meters line was placed on the sidewalk using snow (Figure 4), where the students could walk numerous times counting steps, until an average step size could be calculated.

Figure 4 – Calculating Distance for Pace Count with Laser Device

In the second week, the class could then go to The Priory for the traditional navigation. The points’ coordinates were given and plotted in the map (Figure 5). The class was instructed on how to use the compass, both to take the azimuth value from the map and how to navigate with it.

Figure 5 – Point plotting and azimuth taking using the compass.

A table was made containing initial point, final point, azimuth value and distance between the points. Although it would be useful to have the actual distance between the points in “step” units, accordingly to the pace count of the person who would be the walker; time didn’t allow these measurements, so the distance was actually being taken during the navigation itself.

Then, in groups of three, one would calculate the azimuth with the compass, the second would be the target for the compass, since the trees were too similar to be used as a reference, and the third would walk counting steps. Also, the frequent analysis of the map would provide recognition of the place the group was.

For the third week, the only resources for the navigation would be an Etrex GPS unit, along with a table with the coordinates. To find the points, there are two techniques: one way is to constantly look at the given coordinates on the GPS and observe how they change while you walk. Then, you can fix the X coordinate by walking in a certain direction, and then the Y coordinate in a different direction. However, this method is not very efficient, since you don’t take the fastest way. Then, to go around this problem, a tool in the Etrex unit could be used. There, you input the coordinates of the point you want to get, and then a compass in the screen shows the direction, as well as the distance left. This is pretty in handy since the distance and direction is automatically updated as you walk, so it’s possible to take easier paths instead of being inside the woods all the time.

For last, the fourth week consisted on the navigation with the Etrex GPS unit and a supporting map. Then, the map production was done again, including new features like course points – which were used to create also the lines between them – and the restricted zones, where the use of paintball equipment was not allowed.

The idea is that the groups would try to slow down others by attacking them with the paintball equipment, the penalty for being hit was two minutes stopped for the entire group. The first group who took all the course points would win. Despite the game perspective, the idea was to test the efficiency of the use of a GPS unit along with a supporting map, even with the weather challenges and rival groups.

For both exercises including the GPS unit, the track log was turned on during the activity to represent the path taken by each individual. Although in the first week no pattern was established for the collection, all the units should be set to collect point features every 30 seconds in the final week. The analysis of these paths could show interesting elements.

Discussion

During the exercise, some issues were faced and corrected or understood, allowing the group to use this experience to avoid the same problems in the future.

                Data Source Information

At the first step – map production – one of the features – the two feet contour line, obtained by UWEC survey – didn’t have a defined projection. That means that the features contain coordinates, but the coordinate system referred to this coordinates is not attached to the file. Therefore, depending on the current projection applied to the data frame, the feature will be located on-the-fly, accordingly.

However, if the data frame projection is not the same as the feature, the on-the-fly will locate the feature far off the correct place. In the situations, it’s necessary to first analyze the data source, where the appropriate coordinate system should be available. If not, an important troubleshoot method is to analyze the information on the feature extent (Figure 6) and compare the possible units and distance to the main coordinate systems used: Geographic Coordinate System, Universe Transverse Mercator, State Systems, State Plane Systems and, in some cases, even County Systems. Special attention should be taken on the different datums: even after finding the correct coordinate system, the use of an incorrect datum can place the feature far off.

Figure 6 – Extent of the two feet contour line feature.

During the troubleshooting, it’s essential to pay attention to the tools used to test, “Define Projection” should always be used and not confused with “Project”. The first tool will simply label the coordinates with a coordinate system, while the second will change the coordinates accordingly with the projection chosen. It is best practice to work with features in the same coordinate system, so the project tool should be used later, however, a feature can only be projected after it is labeled.

                Navigation Coordinate System: UTM vs. GCS

The maps were first produced in the Universe Transverse Mercator (UTM) coordinate system with a 20 meter gridline. However, the first activity consisted in using the map along with a compass; therefore, the appropriate coordinate system for this specific purpose should be the Geographic Coordinate System (GCS).

The compass points to the true north, which only GCS has. As noticed in the Figure 7, maps in UTM have parallel gridlines equally distant, because it’s a projected coordinate system. In the real world, the closer you are from the pole, the closer the gridlines should be from each other – which happens in the GCS. Since the area of interest is small, the difference is almost indistinguishable; but it’s important to be aware of this problem because it can have complicated consequences when dealing with large distances.

Figure 7 – Gridline difference between UTM and GCS.

                Compass Trust

Since it is an old school technique and the GPS took its place everywhere, the compass is commonly put in doubt by the ones who are used to other techniques. For that reason, the group couldn’t find one of the points in the first activity.

The reasons for doubt were legit: there was, indeed, a lack of precision depending on how many times the group would stop, because if one error is done in the beginning, it’s carried on with others, accumulating. The magnitude of the error was misinterpreted: this kind of error would take the group something like one or two degrees of the track, which in a small distance doesn’t mean much.

Therefore, it’s necessary to trust the compass and not to exaggerate possible errors while navigating; they do exist, but wouldn’t compromise the activity. It’s important to find a balance of precision awareness. Of course it’s important to be precise and find the right locations, but when you get too worried about small errors, it might cause more confusing than be a helpful attitude.

                Contour Lines Interpretation

After not finding the point with the compass and starting thinking on a direction errors, the map was used to analyze the features surrounding the group. Since the area was mostly full of trees, the best reference was the elevation.

The group was close to a ravine, so the contour lines would help to find the point. However, due to a quick analysis of the map, the contour lines were misinterpreted. The point was located on the bottom of the ravine, but the group was certain that it would be on the top of the ravine. A later analysis of the maps allowed to notice that the map was actually showing the bottom, not the top (Figure 8).

Figure 8 – Ravine analysis by contour line.

The contour lines were part of the map to support the area identification, and they would be really useful if the correct analysis was made. A lesson comes with that: only put information in a map if the reader is able to understand and interpret it correctly, otherwise it can be more confusing than helpful. The users in this case were knowledgeable about the interpretation of contour lines, a more careful reading was necessary though. However, it's important to understand this idea in general contexts, other than this activity: if something is supposed to be released for the public, it might not be a good idea to insert technical concepts and features.

                Weather Preparation

As said before, the activities took place during a cold winter, inside densely wooded vegetation where the snow depth was commonly higher than 50cm. In these situations, it’s essential to have the appropriate preparation. A number of layers are crucial to keep the temperature acceptable, but need to be thought in how it can limit your movement as well. The use of long boots and water-proof is also very useful because the snow can easily be melted and compromise even more how cold the individual will feel. Gloves are also extremely important, not only for the cold: since the trees don’t have their leaves, the branches can easily hurt your hand if you’re not protected.

Results

In the first activity, one hour was used for plotting points and taking azimuths, and the other two hours were only enough to find three points in the first course. As mentioned, the lack of effective in this case was not due the use of compass and map, but due the misinterpretation in the map reading and in the compass doubt. However, even if that was not present, this method is, indeed, more time demanding than others because it’s necessary to stop frequently.

For the second activity, in less than two hours it was possible to go through all the points in the second course and the exercise felt much more smoother than the first one, especially because of the use of the GPS function where you input the coordinates and it will automatically update the direction and distance you need to go. The track logs show how the group could take more pleasing paths with less vegetation and hills (Figure 9).

Figure 9 – Group track logs in the second activity

Lastly, the use of a map in the third activity improved even more the navigation. In this case, 10 points were found in an interval of approximately two hours and a half. The group missed five points, which was more related to the time consumed in the conflict zones, other than because of the navigation method. As it can be seen in the Figure 10, the circled regions have a higher amount of points and represent the times where our group found another group, resulting on a reasonable amount of time shooting until one of the groups would be out for two minutes.

Figure 10 – Third Week: Individual track log and conflict zones.

The same pattern can be noticed on the Figure 11, where the same areas contain a high amount of points from the entire group. Despite these conflict zones, the path taken by the group can be considered reasonable, since it wasn’t necessary to go back and a high amount of points was still covered.

Figure 11 – Third Week: Group track logs map.

When putting all the track logs together, for the entire class (Figure 12), it’s noticed that everyone could reach a high amount of points, if not all of them. Then, it’s possible to say that the most effective way to navigate was with a GPS unit and a reference map. However, it’s important to understand that it was the third time that the class went to the priory, so the place was already not that unknown, which surely support the navigation: it’s always easier to find yourself when the place is familiar. It doesn’t change the high efficiency in this case, but it’s an important element to keep in mind.

Figure 12 – Third Week: Class track logs map.

The presence of a map during navigation is surely helpful and increases the efficiency during navigation.  However, it depends on how the reader can take advantage of this resource: the elements in the  map have to be understandable for the reader and he or she need to have the necessary background knowledge in how to read it. If these elements are found, the map improves immensely the navigation, not only in this context, but in all the other day-a-day situations mentioned before, where navigation take place.

Conclusion

The project provided a rich experience both in technical knowledge as well as in field practice. It was incredible how many technical elements needed to be understood in favor to have a productive navigation activity. It was possible to improve troubleshooting skills, as well as a huge amount of self-evaluation which potentiate the learning obtained, by understanding the reasons for each issue faced. The experience in dealing with challenging weather conditions was also very important to focus the preparation section of any field work. Map making was also a big part of this project, which allowed exercising cartography and GIS skills.

By comparing all the methods for navigation, the use of a GPS is much more effective than the compass, but its precision can be compromised in locations where a compass wouldn’t. Then, the choice of which method is appropriate will depend on the purpose of the project and the area of interest.

Even though the GPS can have its precision compromised, the exercise took place in cloudy days inside a very wooded vegetation – a typical scenario where the PDOP gets higher and the accuracy goes down – and even though it worked very well. Then, in most of the situations, the use of GPS will provide enough accuracy and efficiency, especially with the technology improvements that keep happening in a high rate. Therefore, since a reference map is extremely helpful, the best method in most of the situations will be the use of a GPS unit along with a map.

References

UWEC. The Priory. Available in http://www.uwec.edu/Chancellor/priory.htm. Accessed on April 1^st, 2013.