Realities of Satellite Interpretation(The things that will drive you crazy!) Rachel M.K. Headley, PhD USGS Landsat Project
Interpretation Challenges • Very different land use or land cover types will look the same to the sensor. • There is no “forest” button. • Applies to Supervised and Unsupervised classification • The same land cover or land use type can be classified as unique. • Geography matters! • Context of the image will give clues to solving above mysteries (mostly). • Recognize where you cannot overcome limitations.
Confused classification • Central PA, 6 May 2007 • Golf courses & early green fields • Deciduous forest & bare fields • Coniferous forest more distinct
Confused classification • Near Seattle, 14 May 2007 • Definition of “forest” important • Both Deciduous and Coniferous forests here • Notice the power line cut. • Aspect, slope, sun inclination • Different shadowing can result ‘false’ interpretation • Clearcut looks the same as ag field or developed • Here ag fields & developed lands are on a much smaller scale than timber harvesting. • Density of trees – what do you do with clearcut regrowth?
Confused classification • Southern Missouri, 24 May 2007 • Much greener than previous images – seasonality must be taken into account. • Deciduous is nearly indiscernible from green ag fields. • Are different colors of green different forest types – do you care? • Wet ag fields – what do you call them? • Different signature in bare ag fields.
Confused classification • Yellowstone, 16 May 2007 • Nearly same date as all previous scenes • Same band combination • Higher latitude and elevation = winter scene • Worthless for interpretation! • (Gorgeous though it may be)
15 May 2007 5,4,3
15 May 2007 7,5,2
Confused classification • Northern Virginia, 15 May 2007 • Different bands can tell different stories • Recommendation: create classifications with all bands, except thermal (and pan on L7). • All green things will be confused with each other in 5,4,3. 7,5,2 gives you much more distinction. • 7,5,2 makes bright things too bright. 5,4,3 gives better distinction. • Recommendation: Pick a different date for this area! • Too far into the growing season • Note: Mines: can be mixture of bare soil and water • Regionally dependent
Know what questions you’re asking • Be clever about choosing images • Know what types of land use or land cover you care about • Winter (leaf off): not useful • Early spring: target early greenup • Crop types & forest types (general) • Peak of green • Max NDVI, clear cuts, barren lands • Late fall: target post-harvest • Crop fields from forests
Geography Matters • Changes between years • Similar seasonality • Same dates don’t always work (take 2008) • Know your geography • Rules change • Seasonality • Interpretation • Land cover or land use types • Scale of change • Type of change • Completely unknown study area is doable, but difficult