Lab 17: Measuring and Analyzing Slope

Katie Burles and Crystal Huscroft

In this lab you will be doing a combination of field and office-based analyses of topographical slope. In the field, your technique will simulate a quick low-cost method of approximating slope, and your office-based analysis is typical of preliminary investigations.

Slope is a measure of change in elevation over a known horizontal distance. Often it is used to describe the steepness of a landform surface. One might argue that slope is one of the most significant landscape metrics for geographers to evaluate.

Much of Earth’s surface is sloping, not flat, and as a consequence there are a range of hydrological and geotechnical processes that are activated by the difference in between one location and another. Slope processes bridge several scientific fields such as geomorphology, soil science, hydrology, and engineering.

In this lab you will gather your own field measurements at a location of your choosing, and learn the basics of taking high-quality field notes to support your field measurements.  Then you will input your field measurements into Google Earth (web) to help calculate slope.  Finally, you will measure the gradient of a ski run at a resort of your choosing and plot the vertical profile.

Learning Objectives

After completion of this lab, you will be able to:

  • Record complete, well-formatted and well-organized field notes;
  • Perform a basic surveying measurement in the field;
  • Measure elevation and distance using online GIS platforms;
  • Calculate slope gradient using a spreadsheet program;
  • Generate slope profiles using a spreadsheet program;
  • Relate field observations of gradient to calculated values of gradient.

Pre-Readings

Why Is Slope Analysis Important to Geographers?

Important applications for slope analysis include describing landforms, watershed modeling, characterisation of wildlife habitat, assessing slope stability and mass wasting hazards, classifying soil development, modeling wildfire risk, and assessing potential for land use development.

Recording Field Notes: General Guidance

In the field you must be very neat and organized in recording information. In professional settings, notebooks are employed as evidence that you used proper procedures and conducted yourself professionally by collecting all the relevant information.  Occasionally, field notes are entered into legal proceedings as evidence in support of proper professional processes.

It is important to start early in one’s career with taking well formatted, complete, and proper notes. Field notes will be the only recorded evidence of what you saw and did in the field; there is a saying, “if it is not in your field notes, it never happened”.  Moreover, human memories are surprisingly unreliable, and it will take only a matter of a few days before you have completely forgotten the details of what you did in the field.  Field notes are invaluable in this regard.

In professional settings, erasing mistakes is regarded as “tampering” with field notes. As such, you may not erase mistakes or cover them with white-out. Instead, neatly cross out any incorrect measurements. You never know if or how your observations and measurements will be useful later.

Surrounding Information

In the field, you will be recording GPS position, vertical distance measurements, and photographic images. However, before you take these measurements, there are other important pieces of information to be recorded. Surrounding information is always included so that the measurements become meaningful to other colleagues, employees, or researchers that read your values. This surrounding information includes:

  • Location (coordinates and verbal description of relative position to major landmarks nearby)
  • Date & time
  • Participants (include who is in the field and their roles)
  • Weather (as the weather will influence the quality of your observations, measurements, and notes)
  • Describe your overall goal for this field stop

Photographs

You will be taking photographs for this assignment and they will need to be recorded (remember, if a record of taking a photo is not in your field notes, the picture was never taken). If you take a photo, the following details should be included:

  1. File number (time of the photo is fine if you are using a cell phone)
  2. Direction the camera was pointing
  3. If there is an object for scale so that the viewer of the photograph can tell how large items are in the photo.
  4. The subject of the photograph and any other information that is important for understanding why you took the picture.

 Field Measurements of Elevation Change (Vertical Distance) Using Levelling

As previously mentioned, slope is a measure of change in elevation over a known horizontal distance.  A sighting level is a device that allows a user to be able to aim their view along a true horizontal line. Using a sighting level and a known eye height to measure the change in vertical elevation between two points is an ancient technique that early civilizations used to design aqueduct and irrigation systems.  It is also a modern technique used in professional settings, except with much more sophisticated digital surveying equipment.

We will be using GPS readings of horizontal position built into a smartphone to determine the change in horizontal position. So, you might wonder, why not use the GPS to determine the change in vertical distance too? Although handheld GPS-based measurements of horizontal position are typically accurate to within roughly 10 m, errors in vertical position are typically 2-3 times greater. Therefore, using a level to determine vertical distance can improve approximate measurements of slope over using hand-held GPS measurements alone.

As seen in Figure 17.1, if a level is used to sight a series of target locations on the ground (Point x1 through to Point A’), and the level’s elevation above the ground (y) is known, the vertical distance (VD) from A to A’ can be measured. The vertical distance (VD) is also know as the rise of the slope from A to A’.

 

Figure 17.1. Sketch profile of the leveling methodology. The surveyor, with a sighting height of y, progresses up slope from A to A’ with intermediate stations x1, x2, and x3. The vertical distance is the sum of the number of sighting heights (y). Source: Crystal Huscroft, CC BY-NC-SA.

No matter the type of level that you use, there are several steps that are common to all levelling measurements of elevation change:

  1. Measure the height between the ground and your instrument (y)
  2. Start downhill and work your way uphill.
  3. At the starting point A, sight along the edge of your level, as though you were aiming an arrow with precision toward the slope in order to locate your next sighting position (x1). If you have a field partner, they can landmark the position of your target (x) for you.

Using a GPS to Record Horizontal Position

The Global Positioning System (GPS) uses the relative distance of a ground-based receiver to a constellation of satellites to triangulate and locate the receiver’s position on the Earth’s surface. Almost all cell phones are equipped with GPS-receiving technologies and they can be used in a fashion similar to a hand-held GPS receiver that is designed for terrestrial or marine navigation. Handheld systems are convenient to carry, give rapid readings with reasonable accuracy, and allow the user to record digital information about way points and paths.

As mentioned previously, handheld GPS-based measurements of horizontal position are typically accurate to within roughly 10 m under ideal conditions.  However, errors in vertical position are typically 2-3 times greater.  Ideal conditions include an open view of the sky without blockage due to buildings, bridges and trees so that the receiver can obtain signals from as many satellites as possible.  A more in-depth description of GPS is described in the pre-readings to Lab 14 of this manual.

Slope: Post-Fieldwork Calculations of Gradient

Your fieldwork will involve collecting data in order to analyze slope. After recording your field measurements, it is time to crunch some numbers. The gradient of a ground surface is calculated by the difference in elevation between 2 points on a slope (rise) divided by the horizontal distance between the 2 points (run).  In this lab, measurements of rise and run will be done using a combination of levelling and GPS measurements.

After obtaining the vertical elevation change (rise) and the horizontal distance (run) between two points, gradient can be calculated and expressed in one of three ways:

  1. In elevation fall per slope distance (m/km); i.e., elevation (m) (rise) / distance (km) (run)
  2. As a percent (%)
  3. As an angle (degrees; Figure 17.2)

 

 

Figure 17.2. Slope calculation. Source: Crystal Huscroft, CC BY-NC-SA.

 Example: If a slope rises 100m over a 1 km distance, its gradient is 100 m/km, 10%, or 5.7 degrees.

Low gradient slopes are almost flat and have very little slope, whereas high gradient slopes indicate a steep slope.

Lab Exercises

This lab includes two exercises that result in work to be handed in via PDF.  The submitted assignment will include a series of required figures with captions.

Exercise 1 is partially field-based and should take 1.5 hours to complete once you have found an appropriate field location (excluding travel time).

Exercise 2 is office-based.  This exercise should take 1 hour to complete.  The length of time allocated to this exercise will depend on familiarity using Google My Maps, iMapBC, and Microsoft Excel.

Required materials:

  • Tape measure or ruler
  • GPS equipped cell phone (See your cell phone’s user guide.  Almost all new phones are equipped with GPS capabilities.)
  • Material for a sighting level (see Appendix B)
  • Pencil and field book or paper and a hard surface (clip board) to write on.
  • Camera (cell phone is fine)

Required software:

  • Google Earth (web) – not mobile version
  • Spreadsheet software (Excel, Numbers, or Google Sheets)
  • GPS App that allows confirmation of communication with enough satellites. Avenza Maps is free. GPS Diagnostics is recommended.
  • Mobile levelling app for a phone/tablet or a level constructed using the instructions in Appendix A.
  • iMap BC (No user name is required.)

EX1: Determining Slope in the Field and Using Google Earth

Safety: Do not complete this exercise until receiving a safety briefing and filling the proper paperwork (if applicable) from your lab instructor. In addition, your first goal when you arrive at a site is to ascertain whether the terrain is safe.  

Step 1: Choose an Easily Accessible Slope.

Choose a slope that you can easily visit. This slope needs to be relatively steep, open (no obstructions to your view from the top to the bottom of the slope), and at least 100 m long. Roads sections with safe sidewalks and no intersections are appropriate. Grassy hillsides or open and straight sections of trail are also good.

Step 2:  Measure Your Eye Height

Wearing the footwear you will likely wear during fieldwork, measure the elevation of your eyes above the ground when looking level. If you do not have someone that can help you, or if you do not have a tape measure, stand beside a door frame and lightly mark the position of your eye height with a pencil. Use a ruler or tape measure to measure the elevation of your eyes.

Step 3: Construct Your Level

Using the instructions in Appendix B, build your sighting level.

Step 4: Travel to the Field Site and Start Field Notes

After choosing a slope to measure, walk to the base of your slope. You will call this location Point A.

Using Worksheet 1 and referring to the instructions regarding recording field notes in the Pre-Reading, start the introduction to your field notes.

Step 5: Plan and Document Your Transect (Figure EX1.1)

Plan where you will start and approximately where you will end your slope measurement (location of Point A and Point A’). If you are using an open slope, it is best to travel directly up-slope (i.e., along the steepest trajectory) and not across the slope.

Document this plan by taking a photograph with your downhill start point (A) in the foreground and your approximate end point (A’). This will be your Figure EX1.1 (see the example in Figure 17.3). Make sure you have recorded the subject of the photo, the time the photo was taken, the direction of view, and the item for scale as described as described at the beginning of this lab.

Figure 17.3. (Sample) EX1.1. Photograph of entire slope used in Exercise 1. This is a photograph looking southward up slope from Point A to Point A’. Source: Crystal Huscroft, CC BY-NC-SA.

Step 6:  Document Point A. (Figure EX1.2)

While standing at Point A, record an accurate GPS reading by ensuring that you have at least 4 satellites. This may take a few minutes.

Include a verbal description of where Point A is located so that you will be able to tell if your location maps incorrectly into Google Earth.

Take an informative photograph of Location A. This photograph will be Figure EX1.2 of your report (see the example in Figure 17.4). Include the information you collect in this step in the figure caption.

Figure 17.4. (Sample) EX1.2. Location of Point A. Looking north. Photograph of Point A (base of red arrow) looking northward. Source: Crystal Huscroft, CC BY-NC-SA.

Step 7:  Level Up Towards A

Standing at Point A (the base of your slope), use your level to landmark the position on the slope that is perfectly horizontal to your eye height (x1 in Figure 17.1). Walk to your landmark (x1) and repeat sighting to the next landmark position on the slope that is perfectly horizontal to your eye height (x2).

Repeat and record the number of intermediate landmarks required to level up until your sighting is landmarked above your desired endpoint (i.e., where you would overshoot your desired endpoint.) This last position will be your actual endpoint (Point A’).

Step 8:  Document and Describe A’ (Figure EX1.3)

While still standing at Point A’, record an accurate GPS reading by ensuring that you have at least 4 satellites. This may take a few moments.

Include in your notes a verbal description of where Point A’ is located so that you will be able to tell if your measured location maps incorrectly into Google Earth.

Take an informative photograph of Location A’. This photograph will be Figure EX1.3 of your report (see the example Figure 17.5).  Include the information you collect in this step in the figure caption.

Figure 17.5. (Sample) EX1.3. Location of Point A’. Photograph of Point A’ (base of red arrow) looking northward. Source: Crystal Huscroft, CC BY-NC-SA.

Step 9: Photograph Your Field Notes (Figure EX1.4)

Once you are done your field work, and preferably before you leave the field, take a picture of your field notes in order to create a backup and include them in your assignment (see example Figure 17.6).

Figure 17.6. (Sample) EX1.4. Field notes. Photograph of field notes taken on June 17th and 18th, 2020. Source: Crystal Huscroft, CC BY-NC-SA

Step 10: Determining Horizontal Distance Using Google Earth (Figure EX1.5)

Once back from the field, start a new project in Google Earth (Web) and name it <Lastname Firstname Student Number L17 EX1>. Press the blue NEW FEATURE dropdown menu and choose SEARCH TO ADD PLACE.

Input the recorded latitude longitude of Point A and click the ADD TO PROJECT button once a placemark has been created.

Repeat the same steps to plot the recorded position of Point A’.

Once both Point A and A’ have been plotted, use the MEASURE TOOL to determine the horizontal distance between Point A and Point A’. Create a screen capture of your horizontal measurement. This image will be used as Figure EX1.5 in your assignment (see example Figure. 17.7).

Figure 17.7. (Sample) EX1.5. Google Earth horizontal distance measurement. Screen capture of the measurement of horizontal distance based on field GPS measurements of the locations of Point A and A’. GPS measurements were made using the datum WGS84. Google Earth imagery uses GWS84 Web Mercator. Source. Google Earth. Used in accordance with Google Earth terms and conditions.

Step 11: Calculate the Gradient

Calculate the vertical distance from Point A to Point A’ by multiplying your eye height by the number of sightings that you made in order to reach A’. Type out you answer and show your work (see example Table 1).

Calculate the gradient in the following ways:

  • Expressed as m/km.
  • Expressed as %.
  • Expressed as angle.
Table 17.1. Example of Table EX1.1. Gradient Calculations
Work (Including Intermediate Steps) Calculation
Elevation change (field measured) =number of eye height measurement *eye height

=6*1.60

=9.6 m

9.6 m
Gradient (m/km) =rise/run

=9.6 m /0.0878 km

=109 m/km

109 m/km
Gradient (%) =rise/run

=100%*9.6 m/87.8m

=11%

11%
Gradient (⁰) =TAN-1(9.6m/87.8m)

=TAN-1(0.109)

=6.2⁰

6.2⁰

Step 12: Assemble a PDF of Exercise 1

Collect your photographs and table into a PDF file.

EX2: Create a Slope Profile of a Ski Run Hill in British Columbia

Step 1: Choose a Ski Run

Open Google My Maps. CREATE a new map. Using the search function on the left of your screen, visit a ski hill (resort) in British Columbia (Click here for more information about BC ski resorts). Zoom into the ski hill until all the lifts and runs are visible on the map. Select one ski run to use in Exercise 2.

Step 2: Outline the Ski Run Using Google My Maps (Figure EX2.1)

To create Figure EX2.1 for your report, use Google My Map, ADD LAYER and draw a thick yellow line over the ski run selected (see example Figure 17.8). Include a screen capture of the run with the name visible in this exercise. ADD MARKER and record the latitude and longitude in decimal degrees. SHARE the layer and copy the link to include in the Figure EX2.1 caption. Your caption should also include a description including the ski hill name, and coordinates.

Figure 17.8. (Sample) EX2.1. Google My Map screenshot of black diamond ski run named Fuzzy at the Kimberley Alpine Resort located at 49.68 N, 116.04 W (<include shareable link here>). Source: Katie Burles, adapted from Google My Map (accessed June 22nd, 2020). Used in accordance with Google My Map terms and conditions.

 

Step 3: Draw and Describe the Profile of the Ski Run Using iMapBC (Figure EX2.2)

Open iMapBC and launch application (Launch iMapBC). Do not select the option that requires a username. Change the base map from Roads to Imagery. The ski hill lifts and runs are not drawn in iMap, but they are visible as clear-cut areas in the imagery.

Find your ski run. Using the LAT/LONG function in the tool bar, zoom to the run. To input Lat/Long, units will need to be converted from decimal degrees to degrees, minutes, and seconds. Use the National Geodetic Survey Coordinate Conversion and Transformation Tool to convert. Input the Lat/Long in degrees, minutes, and seconds to Zoom into the ski run in iMap.

ADD a PROVINCIAL DATA LAYER using the Data Sources tool. Search layer catalog for Contours – (1:20,000) (Base Maps – Contours – Contours – (1:20,000)).

Create Figure EX2.2 for your report by drawing the ski run using the SKETCH tool. Using the IDENTIFY tool, find the maximum and minimum elevation of the ski run. Using the DISTANCE tool, measure the length of the ski run. Add TEXT to the map, including the name of the run, the run length (ex. Rise = 832.3 m), and the maximum and minimum elevation (see example Figure 17.9).

Figure 17.9. (Sample) EX2.2. iMap screen capture of ski run named Fuzzy at Kimberley Alpine Resort. The ski run is approximately 832.3 m in length drops 300 m in elevation. Top and bottom of the ski run is located at 1900 m and 1600 m, respectively. Source: Katie Burles, adapted from iMapBC (accessed June 22, 2020). iMapBC content: Copyright Province of British Columbia. All rights reserved. Reproduced with permission of the Province of British Columbia.

 

Step 4: Describe the Natural Breaks in Slope for the Ski Run (Figure EX2.3 and Table EX2.1)

By inspecting changes in the spacing of contour lines, divide the ski run into 4 sections at natural breaks in slope. Breaks in slope indicate a change in physical continuity in the slope profile. Sections of your ski run may be steeper (more linear distance between contour lines closer together) than other sections.

Position your breaks in slope where your path (the ski run) crosses a contour line. Measure the horizontal distance between each slope break and record the elevation. This will be Figure EX2.3 in your report. Figure 17.10 is an example outlining four slope breaks selected for the Fuzzy ski run. Create Table EX2.1 (see the example Table 17.2) to include the measurements you collected.

Figure 17.10. (Sample) EX2.3. Four natural slope breaks along the ski run Fuzz at Kimberley Alpine Resort. Source: Katie Burles, adapted from iMapBC (accessed June 22, 2020). iMapBC content: Copyright Province of British Columbia. All rights reserved. Reproduced with permission of the Province of British Columbia.

 

 
Table 17.2. Example of Table EX2.1. Data Collected for Four Natural Slope Breaks Along the Ski Run Fuzzy at Kimberley Alpine Resort
Break in Slope Ski Run Distance between each break in slope (m) Continuous Ski Run Length (m) Elevation Change between break in slope (m) Elevation at each break in slope (m)
0 (Top of Run)   0   1900
1 242.5 242.5 40 1860
2 237.2 479.7 80 1780
3 139.3 619 60 1720
4 (Bottom of Run) 213.3 832.3 120 1600
Total 832.3   300  

 

Step 5: Create a Profile in Microsoft Excel (Figure EX2.4)

Open Microsoft Excel and enter the data in columns similar to Table EX2.1. Figure EX2.4 in your report will be a profile of the slope using a Scatter (x.y) Chart (see example Figure 17.11, Example [Excel]).

Figure 17.11. (Sample) EX 2.4 Profile of the ski run Fuzzy at Kimberley Alpine Resort. Source: Katie Burles, CC BY-NC-SA.

 

Step 6: Calculate Gradient Using Microsoft Excel

Calculate gradient in Microsoft Excel. Use the three calculations provided in the Pre-Reading. Calculating gradient in degrees requires the use of the =ATAN function

Gradient as an angle (degrees) in Microsoft Excel:

(=ATAN(rise/run)*(180/π)). To inputting π in Microsoft Excel, use PI().

Calculate the AVERAGE gradient for each and include in Table EX2.2 (see example Table 17.3).

 
Table 17.3. Example Table EX2.2. Gradient Calculations for the Ski Run Fuzzy at Kimberley Alpine Resort
Break in Slope Gradient (m/km) Gradient (%) Gradient (degrees)
1 165.0 16.5 9.4
2 337.3 33.7 18.6
3 430.7 43.1 23.3
4 562.6 56.3 29.4
Average 373.9 37.4 20.2

Step 7: Prepare Your Lab Assignment for Submission

Open the Word document created in Exercise 1. Add Exercise 2 and include Figures EX2.1-EX2.4 and Tables EX2.1-EX2.2 with detailed captions for each (see Table 17.4).

Save the Word document as a PDF file, and submit.

 
Table 17.4. List of Figures and Tables to Create for Your Lab Report. (Figure descriptions and example figures included.​) Use this table as a checklist to make sure you have all of the necessary elements in your report.
Figure or Table Number Description Caption for Figure (Attribution)
EX1: Field and Google Earth-based determination of slope
Figure EX1.1 Photograph of entire slope used in Exercise 1, include direction and locations of A and A’ See example Figure 17.3 (Photograph Attribution)
Figure EX1.2 Photograph of Point A location See example Figure 17.4 (Photograph Attribution)
Figure EX1.3 Photograph of Point A’ See example Figure 17.5 (Photograph Attribution)
Figure EX1.4 Field Notes See example Figure 17.6 (Photograph Attribution)
Figure EX1.5 Google Earth horizontal distance measurement See example Figure 17.7 (Image Source – Screen capture must include Google logo and third-party data providers)
Table EX1.1 Include elevation change (field measurement) and gradient calculations in m/km, %, and degrees See example Table 17.1
EX2: Create a slope profile of a ski run at ski hill in British Columbia
Figure EX2.1 Google My Map screen capture of the ski run See example Figure 17.8 (Image Source – Google My Maps Attribution)
Figure EX2.2 iMap screen capture of the ski run including the name of the run, 20 m contour lines, maximum and minimum elevations, and run length See example Figure 17.9 (Image Source – iMapBC Attribution)
Figure EX2.3 iMap screen capture of the 4 natural slope breaks you used for your profile See example Figure 17.10 (Image Source – iMapBC Attribution).
Table EX2.1 Data collected for the 4 natural slope breaks you used for your profile See example Table 17.2
Figure EX2.4 Profile of your ski run created in Microsoft Excel See example Figure 17.11
Table EX2.2 Gradient calculations completed in Microsoft Excel for your ski run, including average slope See example Table 17.3

Reflection Questions

Please take 15 minutes to answer the following questions.

  1. Goudarzi and Landry (2017) found that the horizontal positional accuracy of locations using Google Earth in one of Canada’s largest cities (Montreal) was up to 2.7 m off.  In contrast, Mulu and Derib (2019) found that horizontal positional accuracy was up to 11 m off in Cairo, and between 1.5 and 4.5 m off in other African cities.  Given this information, the GPS you used, and considering your field location, how do you think your field measurements of horizontal position would compare with Google Earth measurements of horizontal position? Explain your reasoning in terms of the factors that would affect your field data and Google Earth data.
  2. How precise are your vertical distance measurements, approximately? Give give your estimates in terms of numerical values over a distance of 25 m (E.g., ±4 cm over 25 m). How might you test the precision of your vertical distance measurements? Explain the reasoning behind your answers.
  3. How did your gradient measurement in the field compare to your ski run measurement? Would your field location be steep enough to be an interesting ski run? Explain your answer.

Worksheets

Back to Lab Exercises

 

Field Notes Template

Supporting Material

Appendix A: Building a Level Using an App on Your Cell Phone

Back to Lab Exercises

Materials:

  • Smart phone (recent model with accelerometer)
  • Levelling app
    • IOS devices can use the Measure app installed by default on your phone. Just press ‘level” (bottom of screen).
    • Android phone users may want to try AR Measure
  • An 8.5”x11” piece of scrap paper rolled into a tube approximately the diameter of a straw
  • Tape
Figure 17.12. Rolled paper being used as a sighting tube by affixing it with tape to a cell phone. Note that the screen indicates that a reading of 0 degrees has been reached. Source: C. Huscroft, CC BY-NC-SA.

Instructions:

Tape the paper tube along the long edge of your smartphone, while making sure that buttons do not get in the way of the tube being absolutely parallel with the long edge of your phone.

Using your paper tube, sight along the long edge of your phone as in the photograph above (Figure 17.12).

Appendix B: Constructing Your Own Level

Back to Lab Exercises

Materials:

  • Cardboard
  • Protractor template (see Appendix C).
  • 50 cm of thin string or sewing thread
  • Stapler or tape
  • A weight (such as a bag of coins)
  • Scissors
  • Paper glue
  • Tape
  • An 8.5”x11” piece of scrap paper rolled to the diameter of a straw and taped

Instructions:

  1. Glue the template (see Appendix C) to a piece of cardboard.
  2. Cut along out the template while glued to the cardboard.
  3. Attach the weight to the end of the string.
  4. Knot the other end of the string.
  5. Staple or tape (must be strong) the knotted end of the string to the center of the protractor.
  6. Tape the rolled piece of paper along the straight edge of the protractor.
  7. Level by sighting through the paper tube until the weighted string aligns with the 90⁰ line.
Figure 17.13. A level constructed with the protractor in Appendix C, cardboard, a string with a weight, and rolled paper being used as a sighting tube. Source: C. Huscroft, CC BY-NC-SA.

Appendix C. Protractor Template

 

Click the image below to download.

Figure 17.12. Printable protractor. Source: Scientif38 (2010) Public Domain. View source.

References

Goudarzi M.A.& Landry R. Jr. (2017) Assessing horizontal positional accuracy of Google Earth imagery in the city of Montreal, Canada, Geodesy and Cartography, 43(2), 56-65, DOI: 10.3846/20296991.2017.1330767; https://www.tandfonline.com/doi/abs/10.3846/20296991.2017.1330767

Mulu, Y.A., Derib, S.D, 2019. Positional Accuracy Evaluation of Google Earth in Addis Ababa, Ethiopia Artificial Satellites 54(2). https://doi.org/10.2478/arsa-2019-0005; https://content.sciendo.com/view/journals/arsa/54/2/article-p43.xml

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