It is generally known as an Archaeological North Arrow. It serves several purposes all at once. The archaeologist will put this down in view of the find, oriented to true North. The scale is usually marked in centimeters, so there is a valid way of determining the size or scale of what is being photographed (many people might use something like a dollar bill in a crunch). The white portions can be written on with a marker to give information (not needed in this case because of the board) and the North arrow will be printed in some defined color quality, so that contrast can be determined (so that the background lighting can be qualitatively determined (which will help in determining things like shadows and lighting/coloration)).
If it is a well known and already documented dig, the reference board will have location identifiers based on the grid-layout of the site. If it is a brand new find, the archaeologist might put the GPS coordinates on the board, or on the North Arrow directly, again, in order to help identify the exact location of the find.
As it happens, this photo appears to be from the EISP, or Easter Island Statue Project. If you go to the EISP.org site and look in Phase 1 Season 2, Photo 9 of 12 shows their use of the same pair of North Arrow and a Letter Board. In Tyler Durden's answer he mentions a Scale Stick. In Phase 1 Season 3, Photo 10 of 12 as well as Photo 12 of 12, you'll see a Scale Stick being used. I haven't linked the photo's, because of potential copyright issues as well as I can't quite figure out how to do it with these photos. Although I haven't found your specific photo yet, it's probably in one of the books.