The total number of times people are likely to notice an ad on an OOH display. Gross impressions are those delivered against a demographic audience for an advertising schedule. In-Market impressions are the average number of times people that live in a defined market (e.g. a DMA or CBSA) are likely to notice an ad on an OOH display. In-Market Impressions exclude Impressions derived from people who travel into or through the market, but live outside of it. In-Market Impressions are the audience from which GRPs are calculated.