To generate a description of a raster map image that includes a representation of a body of water having a color gradient that appears constant in response to a zoom operation, a description of geometry of the body of water is generated for a selected geographic area. Using a raster representation of the body of water having a color gradient, at least two blur raster images are generated: a first blur raster image is generated using a first blur radius, and a second blur raster image is generated using a second blur radius different from the first blur radius. The description of the geometry of the body of water, the first blur raster image, and the second blur raster image are provided to a client device for generating raster map images of the selected geographic area.