原始线要素类为地理坐标系，如何获取以米为单位的距离？

可以使用IPolycurveGeodetic.get_LengthGeodetic();方法，将要量测的IPolyline转为IPolycurveGeodetic即可。
测试代码如下：

            IPolyline polyline = feature.Shape as IPolyline;
         
            IPolycurveGeodetic polycurve = polyline as IPolycurveGeodetic;
            ILinearUnitEdit linearUnitEdit = new LinearUnitClass();

            //Define the properties for the linear unit 
            object name = Meter;
            object alias = Meter;
            object abbreviation = M;
            object remarks = Meter is the linear unit;
            object metersPerUnit = 1;
            linearUnitEdit.Define(ref name, ref alias, ref abbreviation, ref remarks, ref metersPerUnit);
            ILinearUnit pLU = linearUnitEdit as ILinearUnit;

            double distance = polycurve.get_LengthGeodetic(esrType.esriGeodeticTypeGeodesic,pLU);