值为栅格图像左上角像素中心在地图上的X，Y坐标，其值可以根据特征点在地形图中的坐标量测获得。

C or F value, the X and Y coordinates of the center of a pixel at the up-left corner of the image, can be measured according to characteristic point in topographic map.

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指定顶部的位置，以像素为单位。这个值是相对于屏幕的左上角。该值必须大于或等于0。

Specifies the top position, in pixels. This value is relative to the upper-left corner of the screen. The value must be greater than or equal to 0.

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