## Where are you surveying?

### Comparison of heights measured with GPS and heights from GoogleEarth

Introduction

The data were used for this comparison were taken from the measurement lines of the project. Comparison was made for an even and uneven terrain, as the results would be as objective as possible.

Comparison heights for even terrain

Comparison heights for even terrain was based on a sample of 145 points.  For each point were measured by X, Y and Z coordinates with GPS. After that,  X and Y coordinates are transformed from the coordinate system of the state to the WGS 84, and then it was done converting .shp file to .kml file, so that the measured points could be seen on GoogleEarth. For each point is stored attribute "measured height", which is shown with the corresponding point Figure 1 or on an interactive map http://milanbozic.weebly.com/poredjenje-visina-ravnomjeran-teren.html.

Figure 1: Sample points for even terrain

Maximum height measured with GPS is 128.73 meters (point number 135), and maximum height from GoogleEarth is 127 meters (point number 134, 137, 138).

Minimum height measured with GPS is 114.56 meters (point number 1), and minimum height from GoogleEarth is 111 meters (point number 1).

The difference between the maximum and minimum height measured with GPS is 14.17 meters, and the difference between the maximum and minimum height from GoogleEarth is 16 meters.

The maximum deviation is 4.18 meters at the point number 129, and the minimum deviation is 0.00 meters at the point number 83.

Figure 2 shows a graph of the heights measured with GPS and heights from GoogleEarth, for even terrain. The blue line represents the shape of the field, obtained from the heights measured with GPS and the red line represents the shape of the field, obtained from the height from GoogleEarth.

Figure 2: Heights chart for even terrain

Comparison  of  heights for uneven terrain

Comparison of height for uneven terrain was done in the same way as for an even terrain, only in this case the comparison was carried out on a sample of 57 points.

In Figure 3 are shown the points that were used for comparing height for uneven terrain, points also can be seen on the interactive map http://milanbozic.weebly.com/poredjenje-visina-neravnomjeran-teren.html.

Figure 3: Sample points for uneven terrain

Maximum height measured with GPS is 463 meters (point number 48), and maximum height from GoogleEarth is 456 meters (point number 55, 56, 57).

Minimum height measured with GPS is 387.35 meters (point number 1), and minimum height from GoogleEarth is 379 meters (point number 1).

The difference between the maximum and minimum height measured with GPS is 76.65 meters, and the difference between the maximum and minimum height from GoogleEarth is 77 meters.

The maximum deviation is 11.20 meters at the point number 27, and the minimum deviation is 0.06 meters at the point number 51.

Figure 4 shows a graph of the heights measured with GPS and heights from GoogleEarth, for uneven terrain. The blue line represents the shape of the field, obtained from the heights measured with GPS and the red line represents the shape of the field, obtained from the height from GoogleEarth.

Figure 2: Heights chart for uneven terrain

Conclusion

Based on previously published data and analysis can be concluded that the data from GoogleEarth can only be used for rough planning, or for the purpose of conceptual design.

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#### Thoughts

• Height measurements was performed using RTK method, and I used a geoid model.

• Land Surveyor

Nice...

I echo the comments already posted. Thanks for taking the time to share the data.

• Just curious bro when processing the GPS observations (i guess static) did u use a geoid model or the heights are purely ellipsoidal?

• Land Surveyor

Milan,

Well done!  I was captured by the prose and graphs.  I hope as many laymen as land surveyors read this.

As a professional, I could predict the conclusion before I read anything, but I see that the you proceeded very logically from premise to conclusion.  That is a sign of a well trained thinker, or at minimum, a naturally and instinctively talented writer and presenter.

-Scott

• nice test dear

you can use google earth level just to have an idea .

and i prefer to use civil 3d to convert   between coordinate  system and to import the contour lines from civil 3d

• Land Surveyor

Wonderful and thoughtful analysis. I get so tired of telling people (including those who should know better) that the integrity of this type of data has limits. Thank you for breaking it down so well.

This reply was deleted.

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