diff --git a/README.md b/README.md
index 6dea87840a2dbf54a30a81a3b61ab475b376b368..066980e16b0c1b07a1941480bce3d7e05dee1fc0 100644
--- a/README.md
+++ b/README.md
@@ -27,6 +27,9 @@ spatial_ref_sys |     0.02|        0|
 
 ![tables schema](FACT_bldgs.png "Tables")
 
+Properties in footprints contain a dictionary with height (-1 if missing) and confidence, example:
+>{ "height": -1.0, "confidence": -1.0 }
+
 **All spatial objects are converted to EPSG:4326.**
 
 ---