The earth's gravitational pull is not consistent. To avoid conflicts between regions, the value 9.8 m/s2 was used as the standard gravity. In reality, anomalies are depending on one's location. Gravity anomalies are variations caused by surface type, movement of tectonic plates, and so forth. On the mountainous range, gravity would face a positive anomaly. While in trenches, it would be in a negative gravity anomaly.

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Source: https://climatekids.nasa.gov/10-things-earth/