旅游保险费用看起来似乎是没必要的，但却是必不可少的开支。
Taking out travel insurance may seem an unnecessary expense, but it is a must.
你可以向孩子们解释，植物不仅是制作食品所必要的，而且对于控制洪水也是不可少的。
You can explain to them that plants are necessary not only for food but also for controlling floods.
因此，在摄影测量的作业过程中，有必要对近景摄影机进行检校，对数码相机的标定是一项必不可少的作业过程。
So, in the course of photogrammetry job, it is necessary to calibrate the close shot metric camera, and it is an essential job course to demarcate the digital camera.
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