A method for gathering reference data for use in planning and interpreting infrared surveys for the purpose of detecting and locating underground features, such as tunnels, voids, or manmade devices. Measurements, images, or observations at a site having known underground features are recorded. Recorded details include a combination temperatures at or near a soil surface at multiple points across the site in addition to above surface factors such as shading, weather conditions, and objects or foliage. Analysis of the details recorded from the site having known underground features yields quantitative estimates of the effects of various above and below surface factors on temperatures at or near the soil surface.