what r yellow crosses used for

If the yellow crosses are in a public area they could be something to do with new building developments, new pipes and / or sewers going in, or even something to do with the health of the grass. I'd contact the local council office and ask them if anything is planned for the area. I'd imagine if any type of construction is to take place there should be Public Notice displayed. These are normally displayed on illuminous paper tied around lamp posts asking if anyone has any objections to developments to apply in writing within a certain time. I'd be interested to know the answer to this one.