Have them continue until they can identify it. At that point, they should stop and mark the ground with chalk. They should then approach the item posted until they are so close they can no longer tell what it is. At this point, have them mark the ground with chalk again. Finally, have the students move right up to the item posted and examine some detail of it with a magnifying glass and record what they see.
Students then measure the two distances from the poster.
Your students should discuss what kinds of information they observed from each of the various distances. They should try to frame questions that can only be answered from close up, but not from far away, vice-versa, and some questions that can be answered from a medium distance. Explain that scientists gathering data by remote sensing do the same kind of exercise that your students just did. They figure out just how close or far away the camera needs to be to give them the information they want. Discuss the types of data they think the Landsat 7 satellite will be able to provide to scientists.