Rodrigo E. Principe did a great job for Mosaicking a Image Collection by Date (day) in Google Earth Engine (https://code.earthengine.google.com/20ad3c83a17ca27b28640fb922819208) based the code, I mosaic a whole state temporal images, then I have tried to sample some points on a stacked images. While an error said: "The default WGS84 projection is invalid for aggregations. Specify a scale or crs & crs_transform." The tutorial says "An image which is a composite or mosaic of input images with different projections will have the default projection, which is WGS84 with 1-degree scale, generally, an aggregation at 1-degree scale is not desired or intended, so Earth Engine gives this friendly reminder to provide a complete specification for the output." Is there a better method except for reproject() which is not advocated in tutorial.