Learning systems

The fourth entry is a verdade de referência, and Portuguese has done here what English hardly ever does: it has said the thing plainly. A truth held up for reference, the one an answer is checked against. English reaches instead for « ground truth », and the phrase is not the metaphor about bedrock that most people read into it. It comes from surveying and then from remote sensing, where a photograph or a radar return has to be checked against somebody standing in the field: the measurement made on the ground, as against the one made from the air. The ground is a real place, and the truth is whatever a person found when they went there. Hold on to that picture and two consequences follow. « Ground truth » is what was recorded, not what is right in principle, so it can itself be wrong, and a model trained on it inherits the mistake without a murmur. And English has made a verb of it — to « ground-truth » a data set is to go out and check it — where Portuguese must say verificar no terreno and give up the noun altogether.