3D CAD模型长期保存策略研究

3D CAD模型长期保存困难重重。为此,国际航空工业界推出LOTAR(LOng Term Archiving and Retrieval)项目,旨在建立、测试、颁布并维护3D CAD模型长期保存和获取所需要的标准,目前其主要成果是NAS/EN 9300系列标准。而汽车工业产品数据标准战略联盟通过LTAR(Long Term Archiving&Retrieval of Digital Product Definition Data,LTAR)项目已经形成了一套完整的建议体系,包括四个部分:文件格式、存取流程、保存时间、质量保证。目前,国内还未有国家或行业层面的3D CAD模型长期保存策略。借鉴电子文件前端控制和全程管理的思想,通过3D CAD模型生成时的模型质量控制措施、流转过程中的更改和版本控制以及归档后的物理与逻辑实体控制措施,确保3D CAD模型长期可读、可理解。

Integrating space and time: a case for phenological context in grazing studies and management

In water-limited landscapes, patterns in primary production are highly variable across space and time.Livestock grazing is a common agricultural practice worldwide and a concern is localized overuse of specific pasture resources that can exacerbate grass losses and soil erosion. On a research ranch in New Mexico with average annual rainfall of 217 mm, we demonstrate with a quantitative approach that annual seasons vary greatly and examine foraging patterns in Angus-Hereford(Bos taurus) cows. We define five seasonal stages based on MODIS NDVI: pre-greenup, greenup, peak green, drydown and dormant, and examine livestock movements in 2008. Daily distance traveled by cows was greater and foraging area expanded during periods with higher precipitation. A regression model including minimum NDVI, rainfall and their interaction explained 81% of the seasonal variation in distance traveled by cows(P < 0.01).Cows explored about 81 ha·d^(–1) while foraging, but tended to explore smaller areas as the pasture became greener(greenup and peak green stages). Cows foraged an average of 9.7 h daily and spent more time foraging with more concentrated search patterns as pastures became greener.Our findings suggest that phenological context can expand the capacity to compare and integrate findings, and facilitate meta-analyses of grazing studies conducted at different locations and times of year.