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Ground Truthing: Verify Remotely Collected Data‍

Ground Truthing

Ground truthing assesses the accuracy of remote sensing data including satellite imagery, aerial photographs, or other types of remote sensing methods by comparing it with physical measurements collected at the ground level.

What is ground truthing?

Ground truthing is important because it can help confirm or refute the accuracy of the data. In order to ground truth data, researchers must visit the actual location and physically measure it at ground level.

Then, we compare this information with the data collected from remote sensing to determine its accuracy. Because a small error can lead to significant consequences, ground truthing is one of the first steps in the data collection process.

Esri Landcover 10m

Benefits of ground truthing

As you’ve learned already, ground truthing assesses the accuracy of remote sensing data. Here are some of its benefits:

Trustworthiness – Trustworthiness is another important benefit of ground truthing. If you’re using data collected remotely, you may have questions about the legitimacy of the data. Ground truthing can help you verify the data you’ve collected, which can add to its overall reliability.

Confidence – Confidence in the data you have collected is another benefit of ground truthing. If you have verified the data on the ground, you can be sure that it has a specific level of accuracy. This can give you a greater level of confidence in your data, which can be beneficial for your project.

Correcting errors – Another way ground truthing can benefit your project is by helping you correct errors in your data. If you have verified the data on the ground, you can correct any errors in the data and use the correct version going forward.

Setting up a field survey

QField Mobile

First, we start with a field survey to start the ground truthing process. During this step, you collect data on the ground. Then, this data can then be compared to satellite or aerial imagery.

A field survey can be conducted in many different ways, which can depend on the type of data you are collecting and the scale of your project.

Remember that the main purpose of a field survey is to collect data in the field and compare it to the data collected from the remote sensing imagery.

This comparison helps clear up any discrepancies or errors. Field surveys also help to identify any potential issues that may arise from the satellite imagery, such as cloud cover or other environmental factors.

Benefits of field surveys

Additionally, field surveys provide an opportunity to observe the area in question firsthand, which can help to gain a better understanding of the environment and its features.

Therefore, a field survey is essential for ground truthing, as it allows for the verified and reliable collection of data and enables a deeper understanding of the environment being studied.

The alternative to an in-person field survey would be to use existing imagery sources. Aerial photography provides a bird’s-eye view of the area you are working on, which can make it easier to identify discrepancies between the data collected remotely and the on-site data.

Although having boots on the ground is the gold standard, historical imagery can provide a different context for time, which would not be possible in a field survey.

Verifying remotely collected data

For verifying land cover classification, we produce a land cover accuracy matrix to assess its accuracy. In general, we evaluate the reliability and accuracy of the data with two types of accuracy reports (1) producer accuracy and (2) user accuracy.

Producer Accuracy 

Producer accuracy is a measure that determines how well the land cover predicts the true values of the data from the point of view of the map maker. It is typically calculated by dividing the number of correct classified sites by the total number of reference sites.

Producer Accuracy of Water = 28 correctly classified as water at reference sites / 30 total number of reference sites that are water

The total producer accuracy would be 93.3% for water. The higher the producer accuracy, the better the model is at predicting the true values of the data.

User Accuracy

On the other hand, user accuracy is a measure of how reliable a class is on the map compared to the ground from the user’s point of view. It is calculated by dividing the number of correctly classified instances by the total number of classified instances. 

User Accuracy of Water = 28 correctly classified as water at reference sites / 35 total sites that are classified as water.

In this case, the user accuracy equates to 80.0% for the water class. The higher the user accuracy, the more reliable the model is at correctly classifying the data into its respective classes.

Overall, both are important metrics to measure the accuracy of data using field surveys and ground truthing. User accuracy and producer accuracy generally differ in their accuracy scores.

Real-world applications of ground truthing

We can apply ground truthing to a variety of industries, such as agriculture, forestry, and urban planning. However, it is especially useful in remote sensing. Here are some examples ground truthing:

Agriculture – Farmers often collect remotely collected data, such as imagery or NDVI from satellites, to monitor their crops. By collecting ground truthing data, they can verify the accuracy of their data and check for any discrepancies.

Land Cover – Ground truthing methods usually involve visiting sample sites and taking measurements or photos of the site. The field crew notes any discrepancies between the land cover map and what is observed on the ground. This process can identify errors in existing land cover maps to improve it over time.

Forestry – In forestry, ground truthing helps to assess the accuracy of a digital forest inventory collection. For instance, this may include the health and condition of a forested area. Ground truthing can also provide information on the age and size of trees, the presence of invasive species, the extent of erosion, and the amount of vegetation cover.

TerraPop Landcover

Challenges of ground truthing

As beneficial as ground truthing can be, it can also come with certain challenges. First, you must ensure you have enough time to conduct a thorough and comprehensive field survey. Secondly, you also need to be sure you have the right equipment to conduct a successful survey.

Timing – Timing is one major challenge associated with ground truthing. If you don’t go to the field within the correct timing window, you may not be able to verify all of your data.

Equipment – When conducting ground truthing, it is important to have the right equipment in order to obtain reliable and accurate results. This can include equipment such as GPS receivers, cameras, altimeters, and thermometers. Additionally, when measuring certain features, it is important to have the proper tools to get the most accurate readings. For example, when measuring tree heights and widths, field workers can use specialized tools such as clinometers or terrestrial LiDAR.

Summary of ground truthing

In this article, we provided you with information on the benefits, real-world applications, and challenges of ground truthing. Additionally, we touched on how to set up a field survey and how you can use accuracy assessments.

But is there anything that we missed?  We’d love to hear from you. What are some of the challenges that you experience in your field surveys? And how did you overcome them?  Please let us know with a comment below.