In this activity, students compare two maps of the same location from different points in time. They create a plan to measure the difference in forested area. Students make sense of problems when making a plan to calculate area (MP1). 

The goal of this discussion is to highlight the different plans groups have made and how different approaches could impact their final answers. Consider asking:

• “What is your plan for finding the area? Is there a group with a different plan?” (Decompose parts of the map into regions to find the area. Yes, a different group plans to find the area of the entire region and remove the area of the parts without trees.)

• “Do you expect to have the exact same answer as other groups? How close do you expect to be?” (No, our answers will not be exactly the same. We expect to be within 100,000 square feet.)

• “What are factors that could make your answer more or less accurate?” (how we round our measurements, how we decompose our regions)

In this activity students carry out their plans for finding the forested area in each map. Students reason abstractly and quantitatively when finding the area of each shape (MP2). While it is expected that groups end up with similar answers, differences in measurement can lead to different responses. Students construct arguments and critique the reasoning of others when discussing whether or not deforestation and reforestation occur (MP3).

The purpose of this discussion is to highlight the variability in the answers, including how different answers can still be reasonable and how to decide if an answer is so different it would not be a reasonable solution. It is expected that students’ answers for the areas should be similar but not exactly the same. 

First, poll the class on their answers to the area of forested land in each map. Record and display the responses for each area for all to see. Consider asking:

• “Why do different groups have different answers?”

• “Is there a significant difference between the area of forested land in each map?” How do you know?

• “What could you have done differently to make your answer more exact? Explain your reasoning.”

• “Did your group change your approach from what you planned?”

• “Was it easier to find the whole area and subtract areas without trees or find the areas with trees and add them together? Explain your reasoning.”

• “Do you think reforestation balances out deforestation? Explain your reasoning.”

If time allows after discussing students’ answers, consider displaying a map from Global Forest Change, https://im612.org/GlobalForestChange. Focus on your local area or any other example of deforestation or reforestation, such as the wildfires near Los Angeles. Use the settings on the side of the webpage to compare what the forests looked like over time. Consider discussing:

• ”Does this area show any evidence of deforestation?”

• ”Does this area show any evidence of reforestation?”

• “Did those changes occur naturally or through human actions?”

• “Why should communities care about deforestation in their area?”