How to style a GeoTIFF in Tilemill?

Question

I'm trying to create a "heatmap" style visualization from a number of geopoints and values associated with them. I have been following along with this blog post.

I've got a CSV file of points like the following:

POINT_Y,POINT_X,Value
-41.157785,174.8343481,5
-41.1575385,174.8347111,4
-41.1574106,174.8347659,9
-41.1572644,174.8348964,2
-41.1570742,174.8351378,7
-41.1569067,174.835225,1

I've converted them into a GeoTIFF file with interpolation using gdal_grid. This was the command I ran:

gdal_grid -a invdist:power=3.0:smoothing=1.0:radius1=90000:radius2=90000:max_points=0 -a_srs WGS84 -of GTiff -ot Float64 -l test test.vrt test.tiff 

That seems to work and generates a GeoTIFF file that looks right (I can paste the gdalinfo output, in case that helps)

I'm wondering how I style the points to get something similar to what is shown at the bottom of that blog post? I'm using TileMill to style my maps, and when I add the GeoTIFF to my project as a layer I get a white box that's in the correct place:

But of course I can't see any of the individual points on the grid. I'm fairly sure I'm missing an important step to colourize the points, I'm just not sure what it is!

I don't have any Carto styles applied to the layer in TileMill.

Thanks in advance!

Nik

Answer 1

I've had some success using the gdaldem command. Something like the following:

gdaldem color-relief test.tiff ramp.txt test-color-relief.tiff -alpha

Where ramp.txt is a file like the following:

0% 255 255 255 0
20% 255 247 236 255
30% 254 232 200 255
40% 253 212 158 255 
50% 253 187 132 255
60% 252 141 089 255
70% 239 101 072 255
80% 215 048 031 255
90% 179 000 000 255
100% 127 000 000 255

Those are values/percentages for each of the "stops", then rgba colours.

Basically the gdal_grid command from the blog post is converting the values I'm using to z values for each point (i.e elevation) and then I'm using the command to create elevation maps in order to colourize the GeoTIFF.

I still can't get it to look ideal, but I think that's more down to more tweaking required on the gdal_grid parameters than anything else.

Answer 2

The documentation mentions a Raster Symbolizer but lacks an example.

Supposedly

TileMill has a built-in reference containing all of the Carto options. Click on the book icon at the top-right of the TileMill editor.

(Source)