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The basic topographic survey aims to provide a general idea of the terrain's shape without conducting a detailed, exhaustive study. To achieve this, the terrain is traversed using a mobile phone equipped with GPS software to record latitude, longitude, and altitude. It's important to note that these values can have a high margin of error, with altitude having an uncertainty of more than one meter, as well as geographic coordinates.

Once these values are collected, they can be processed using the 'Topography Application' routine, which, with the assistance of Google Maps data, corrects some systematic errors and allows for a preliminary, cost-effective survey.

The survey presented here is of low quality and is not suitable for future terrain stability studies or for defining the precise location of the house.

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To carry out an initial basic low-resolution survey, you can use various GPS applications available on cell phones. It is important to choose an application that not only estimates latitude and longitude but also provides elevation estimates.

Generally, mobile phones provide two types of values: elevation and altitude. Elevation aims to be an estimate of the height of the location above sea level, while altitude estimates the height at which the phone is located. In other words, if you measure these values on an elevated floor of a building, elevation will indicate the height above sea level of the base of the building, while altitude will indicate the height of the specific department you are in.

It is important to note that these data often have a high error rate:

Latitude and longitude on cell phones have an error range of between 5 and 15 meters, and in the case of using an improved GPS system (A-GPS), this error can be reduced to 2 to 5 meters.

Elevation can be measured with an accuracy of 10 to 30 meters, and in the case of an improved GPS system (A-GPS), this error can be reduced to 3 to 10 meters.

However, these values can often be imprecise, not randomly but with a general trend. They can be corrected using higher-quality data, such as those obtained from Google Maps, for example.

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Other data that can be obtained, especially for correcting GPS data, are the coordinates that can be read on Google Maps. By clicking on the map, you can obtain the latitude and longitude:

The precision of Google Maps is highest when you use the zoom function and can reach accuracy within tens of centimeters.

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To conduct the topographic survey, please follow these steps:
• Traverse the terrain while noting its key features.
• Collect reference points from the surrounding area using Google Maps to link them with the points obtained during the survey.
• Enter the points into the 'Topological Application' window.

The data entry is done by copying the information from the text area below.

The survey should cover all points showing changes in slope and prominent locations. Additionally, it should extend close to the edge to enable linking the points to it.

The point format begins with the following line:

[email protected]

Then, each point is entered, indicating whether it was measured ('m') or obtained from Google Maps ('g'), followed by latitude, longitude, and elevation. Here's an example:

[email protected]
g,-39.928507,-73.556544,30
g,-39.928509,-73.556432,25
....
m,-39.928537,-73.556514,29

where '...' represents other points. It's essential to enter them in the order they were surveyed.

The result can be saved by pressing the corresponding button.

If you'd like to see how the function works, you can click the 'Demo' button, which loads data from the Palos Verdes project.

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When you press the draw button, the points entered in the text area are plotted. If a file has been loaded using the corresponding button, the format will be checked, including the first line, to ensure that only data corresponding to the topographic profile of the terrain is read.

The drawing is generated with the data as entered, including any errors they may contain. In this case, the measured data (the red points) are outside the area where they are supposed to be, both in the lower right-hand corner and in the lower left-hand corner. It is assumed that this corresponds to accuracy problems in measuring latitude and longitude. If the error is assumed to be systematic (a consistent trend across all the data), these data can be scaled based on Google data (blue points) when the second scale button is pressed.

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If you press the scale button, the measured points (red points) are scaled in both directions to fit the frame of the Google points (blue points).

After performing the calculation, the program moves to the next tab (with the option to go back to the previous tab) and shows the blue points again, while the red points are corrected.

Before proceeding, you can capture the image, ensure the background is transparent, and overlay it with the Google Maps image:

This way, you can verify if the route matches the path you followed during the measurement.

To continue, you should now press the new button indicating that triangulation will be performed.

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In the next step, the data is triangulated, which means that the smallest triangles covering the entire surface are sought. This way, a preliminary estimation of the surface between the measured points (red points) and the borders (blue points) is obtained. Once the triangles are calculated, they can be assigned colors based on elevation, resulting in a diagram like the one shown:

Just like in the scaling step, you can overlay this diagram with the Google Maps background. To do this, it's important to make the image semi-transparent to recognize the terrain features:

After triangulation, you can estimate contour lines by clicking the button with the same name.

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After triangulation, you have a preliminary estimate of the surface, which allows you to obtain a first estimation of the contour lines:

Since the transition from one surface to the next is discontinuous, the contour lines have corners that do not correspond to real situations. Therefore, it is advisable to perform a smoothing process, which involves averaging the values around each point. To do this, simply press the corresponding button, which will perform the smoothing process. You can repeat the process as many times as you like because the system will use the last calculated map and apply the procedure again.

The number of times you perform the smoothing process depends on achieving a realistic estimate without overdoing it and starting to lose the characteristics of the structure.

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After performing the necessary smoothing operations, you obtain a contour lines diagram that should closely resemble the actual terrain.

It's important to remember that the number of smoothing iterations should:
• Remove the discontinuities generated by the original triangulation.
• Avoid the loss of characteristic structures, which are not numerical artifacts.

For the latter, it's essential to compare the contour lines with the overall Google Maps view. You can achieve this by overlaying the elevation map with the Google Maps image. To do this, remember to make the image partially transparent, which can be done using various graphic tools or the image manipulation function in 'PowerPoint.'

Finally, keep in mind:

The topography obtained is only an estimation that can guide initial earthwork but is not suitable for soil stability studies or the final house placement in the design.

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