Why do elevation readings differ?
USGS terrain elevation estimates the ground surface. A phone’s altitude is a device measurement. A building floor, trees, satellite geometry, map resolution and vertical reference can make the numbers differ.
Terrain cell size is not height accuracy
The USGS Elevation Point Query Service samples elevation data at a coordinate. The returned terrain resolution describes horizontal cell size. It is not a promise that the height is accurate to that many meters. A finely formatted number does not make the terrain a survey.
Use the right vertical reference
USGS products have documented vertical reference systems that vary by product and coverage. A device altitude may use another reference. Compare only heights whose datum, units and location are understood. The service result can include acquisition information; requesting it today does not make the underlying terrain survey new.
Profiles sample rather than survey a route
A profile connects sampled heights. It can miss a narrow peak or depression between samples. Use it to understand broad terrain, not to design construction, assess flood exposure or navigate hazardous terrain. A topographic map adds contour context, while the official product documentation explains its limitations.