I have LiDAR data as a TIFF file. As expected, human-made structures such as buildings show up as high points. I would like to have just ground-level elevations. I believe that what I want to do is replace the building high point elevations with the elevation of the adjacent points. Could anyone suggest a way to do this? I am using QGIS 3.
Your lidar-derived raster is known as a DSM (Digital Surface Model), which depicts the top surface of objects such as buildings and trees. Most (all?) lidar data collection projects have the ability to concurrently acquire bare-earth data, known as a DTM (Digital Terrain Model) wherein above-ground objects (trees, buildings) are removed. While it might be possible to remove above-ground objects from a DSM, it would be far, far, far(!) easier to acquire the DTM from the data provider, or failing that, create your own DTM from the lidar point cloud.