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Journal of Geo-information Science >

2017 , Vol. 19 >Issue 7: 901 - 908

DOI: https://doi.org/10.3724/SP.J.1047.2017.00901

Orginal Article

A Realization Method of Navigation Electronic Map for Ground Station based on OpenStreetMap and 90 m SRTM

  • SONG Xiaohu ,
  • ZHU Jihong , *
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  • State Key Laboratory of Artificial Intelligence, Computer Science and Technology, Tsinghua University, Beijing 100084, China
*Corresponding author: ZHU Jihong, E-mail:

Received date: 2016-12-09

  Request revised date: 2017-04-27

  Online published: 2017-07-10

Copyright

《地球信息科学学报》编辑部 所有
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Abstract

Navigation electronic map is the only platform for operators to monitor unmanned vehicles. It is also an important information source for route planning as well as auto decision making. A navigation electronic map which covers wide territory and of high precision can provide more details and is an indispensable guarantee for a ground station task. Nowadays, the implementations of navigation electronic maps for unmanned vehicles are mainly based on secondary development on both online and offline map server platforms. These implementations may bring inevitable drawbacks. For one thing, map data and corresponding software such as MapX are very expensive. Even so, the map data always lacks flexibility for personal extensions such as altitude. Besides, the installation and configuration increases much difficulty for the ground station software developers. Finally, online platforms always rely on stable and high-speed Internet network environment, which is not always satisfied. With the development of free and open source geographic data and tools, individuals can easily modify and even construct their own map servers based on their individual needs. In this paper, we provide a new method to implement a navigation electronic map based on a widely crowd sourced map, which is called OpenStreetMap. Firstly, we set up a background tile service using raw OSM data. Then, we render 90 m SRTM data to a bunch of hill-shading and color coding raster files and define the rendering formats and layers in the configure file of the service style for each raster hill-shading and color coding file. Finally, we implement a client module in ground station to request tiles from the background service and present the map in the user interface. The navigation electronic map we implement covers all 0-18 zoom levels of the whole China and it provides 0-13 zoom levels of vivid topographic information and free of Internet limit. Besides, this navigation map we implement can also render other DEM sources and any other raster-format files, such as population, vegetation, precipitation and so on.

Key words： ground station; navigation electronic map; OpenStreetMap; 90 m SRTM

Cite this article

SONG Xiaohu , ZHU Jihong . A Realization Method of Navigation Electronic Map for Ground Station based on OpenStreetMap and 90 m SRTM[J]. Journal of Geo-information Science, 2017 , 19(7) : 901 -908 . DOI: 10.3724/SP.J.1047.2017.00901

1 引言

地面导航是无人驾驶装备地面站的重要功能之一。在导航电子地图中实时准确地显示无人驾驶装备的航迹以及姿态等信息,能帮助地面站操作员判断无人驾驶装备的当前状态,也是操作员给出相关操作指令的重要依据。因此,覆盖地域范围广,信息类型丰富的导航电子地图是无人驾驶装备能够有效完成任务的重要保证。
当前导航电子地图的实现主要基于2种方式：①借助第三方的软件,如MapX、MapInfo等,通过购买特定格式的地图源数据,然后根据这些软件提供的类库进行二次开发[1];②通过Google Earth等在线地图服务提供的API进行二次开发[2]。这2种方式均有各自的缺陷：第①种方式增加了开发成本,数据量一般也较小;第②种方式依赖于快速稳定的网络环境,从而增加了任务失败的风险。
对其他数据类型,尤其是高程数据支持不足也是这2种实现方式的另一大弊端。目前,在无人装备地面站导航地图中整合多种类型的信息是地面站的发展趋势。早在2004年,有学者提出将虚拟现实技术应用在军用无人机地面站导航地图中的构想,他们认为一个三维的虚拟战场环境能够减少地面指挥员从多个导航地图界面中抽取分析有效信息的时间[3]。国内外学者尝试将高程数据处理成三维数据,并成功地将其应用到导航电子地图中[4-5],但是地图范围较小,还处在理论实验阶段。现有的2种实现方式均依赖对应的类库或框架,支持的地图信息种类固定,允许用户添加的自定义图层或样式类型较少,因此很难将高程等其他类型信息整合到导航地图中。
近年来,众包与自发地理信息的兴起和普及已经成为当前地球空间信息学重要的特征之一[6]。在此背景下,人人都是地理信息的提供者,开放的地理数据和开源易用的数据处理软件也使得普通用户设计实现符合个人需求的个性化地图成为可能[7-9]。本文即利用众包与自发地理信息最典型的代表OpenStreetMap以及公开开放的90 m SRTM数据,实现了应用于无人装备地面站的导航电子地图,并成功应用于某型无人机仿真系统中。由于OpenStreetMap数据全部开源,覆盖全球大部分区域,因此开发成本小、数据量大,并且不依赖互联网;同时,本文通过对90 m SRTM数据进行处理,以及对OpenStreetMap的渲染样式进行配置,使得OpenStreetMap支持高程数据的显示,从而克服了现有2种实现方式的缺陷。

2 数据源

2.1 OpenStreetMap

OpenStreetMap（简称OSM）是一个存储海量XML数据的数据库（在本文中简称原始OSM数据）,只要注册账号,任何人均可以对其后台数据库进行编辑,从而被称为世界的维基地图[10]。尽管众源地图的编辑过程难以监控管理,但通过近年来的完善和修订,OpenStreetMap的质量已经得到众多学者的认可[11]。
原始OSM数据使用点（Nodes）、线（Lines）、关系（Relations）以及标签（Tags）来表示地图中的道路、建筑等元素[10],因此OpenStreetMap可以满足普通电子地图的需求,当前也开始逐渐应用于导航和定位等领域[12-13],但是原始OSM数据对高程信息支持不够,一般不包括人文、经济等信息,因此仅使用原始OSM数据并不能满足地形图或其他不同类型专题地图的需求。
原始OSM数据不能直接显示为电子地图,需要经过引擎渲染得到瓦片（一般为栅格图片）,然后按照需求呈现给用户。与百度地图和谷歌地图类似,OpenStreetMap同样使用墨卡托投影（Mercator Projection）[10]。这些瓦片按照放大缩小的层级以及投影的位置坐标存储在瓦片金字塔（Tile Pyramid）的结构中。瓦片金字塔结构的每个层级都有一张或多张瓦片;层级越高,瓦片数量越多,每张瓦片表示的范围也越小,绘制的信息越详细。经纬度标号相邻的瓦片连接在一起即可组成表示一定区域范围的地图。瓦片金字塔的结构使得一定的缩放层级下,每张瓦片和某个特定的经纬度范围存在一一对应的关系[14],本文中导航电子地图的实现即应用了该性质。
如图1所示,5张瓦片均为使用OSM数据渲染的256×256的PNG格式图片,最左边的数字表示缩放层级,中间和右边的数字分别表示经度标号和纬度标号。左边瓦片的缩放层级为0,用来表示墨卡托坐标系下整个世界的全貌;右边瓦片的缩放层级为1,在该层级需要使用4张瓦片来表示整个世界的全貌,每张瓦片表示的范围比左边瓦片要小,但包含的信息更为详细。
Fig. 1 General view of the tile pyramid

图1 瓦片金字塔示意图

2.2 90 m SRTM

OpenStreetMap应用于无人驾驶装备地面站最大的问题在于其本身对高程信息支持不够,由于OpenStreetMap数据主要由参与者通过支持GPS 功能的设备获得,而通过GPS获取的高程和经纬度相比精度较差,误差较大,因此在原始OSM数据中很少保存采集的高程数据[15]。由于Mapnik样式文件支持用户添加自定义图层,为了将高程信息显示在OpenStreetMap地图中,一般将第三方的高程数据处理成OpenStreetMap渲染引擎能够处理的格式,然后将其与原始的OSM数据一同渲染成瓦片[16-17]。
本文利用90 m SRTM高程数据和原始OSM数据搭建了能够提供高程信息的地图服务后台。SRTM全称航天飞机雷达地形测绘任务,其数据是一种典型的栅格类型高程数据 [18]。按照分辨率,SRTM数据可以分为30 m SRTM数据和90 m SRTM数据。目前,中国境内的90 m SRTM数据可以免费获取。

3 无人装备地面站导航电子地图技术实现

3.1 框架概述

本文实现的地面站导航电子地图主要包括瓦片服务后台和地图客户端模块2部分,如图2所示。
Fig. 2 Implementation framework of the navigation electronic map

图2 导航电子地图实现框架

瓦片服务后台可以将原始OSM数据和90 m SRTM数据渲染成瓦片,并通过HTTP协议向地图客户端提供服务。
地图客户端实现分为参数控制模块、瓦片请求模块以及显示模块3部分。其中,参数控制模块负责监听用户的操作或收到的数据,保存并更新飞行状态信息;瓦片请求模块通过接收控制模块的参数计算生成瓦片需求列表,并向瓦片服务后台请求瓦片;显示模块负责将请求返回的瓦片以及飞行状态信息绘制在地图面板上。

3.2 OpenStreetMap瓦片服务后台框架

瓦片服务后台集存储原始数据,处理瓦片请求,管理及渲染瓦片于一体,渲染引擎大都使用Mapnik工具包[19]。图3为瓦片服务后台框架图。
Fig. 3 Framework of the tile server background

图3 瓦片服务后台框架

其中,postgresql/postgis用于存储原始OSM数据。osm2pgsql工具用于将原始OSM数据导入postgresql/postgis数据库。Mod_tile和Renderd用来处理用户的请求,并管理渲染的瓦片。Mapnik是整个瓦片服务后台的渲染核心,通过样式配置文件定义了数据的来源以及每种数据渲染的样式。
本文利用Mapnik渲染生成的瓦片为PNG格式,像素大小为256×256的图片,每张瓦片的存储格式如下：zoom/x/y.png。其中zoom表示目录,对应缩放的层级,一般大小从0到18;x表示zoom下的子目录,对应瓦片的经度;y表示x目录下的png文件,对应瓦片的纬度。因此,每个瓦片都包含3个重要的参数,缩放层级、经度标号和纬度标号。在缩放层级为z的目录中,经度标号范围从0~2z-1,表示从西经180°到东经180°的范围;纬度标号范围从0~2z-1,在墨卡托坐标系下表示从北纬85.0511°到南纬85.0511°的范围;因此整个层级为z的目录中所包含的瓦片数量为2z×2z。
通过HTTP协议,可以根据URI/zoom/x/y.png的网络路径向瓦片服务后台请求某个瓦片,其中URI表示服务器中存储瓦片文件的地址。

3.3 瓦片服务后台的90 m SRTM高程数据渲染

地形地貌可以表示为等高线（Elevation Contour Lines）、地形颜色渐变（Color Coding）以及地形阴影（Hill Shading）等[20-21]。其中等高线可以通过矢量格式保存,占用空间小,读取速度快,并且可以提供较为具体的数值,但等高线的读取判断不够直观。地形颜色渐变可以通过不同的颜色表示海拔,地形晕染不能表示海拔高度,而是通过不同的灰度值来描述地形的起伏,将地形颜色渐变层和地形晕染层进行叠加即可立体化地表示地形地貌。
单个90 m SRTM文件通过格式转换即可处理成地形颜色渐变和地形阴影栅格文件,再经过坐标转换即可投影到墨卡托坐标系。但这样会存在2个问题：（1）经过坐标转换会使相邻文件之间出现由于计算精度误差导致的重叠或空缺;（2）较低的层级下每个栅格文件的细节得不到充分表达,但由于渲染的栅格文件数量太多会导致后台服务效率过低。为了解决这些问题,在坐标转换前先将所有90 m SRTM数据文件虚拟为一个整体,经过重新采样以及坐标转换之后再将其分割,最后进行颜色渐变和晕染格式的处理。具体流程如图4所示。
Fig. 4 Processing flow of color coding and hill-shading raster files

图4 颜色渐变和晕染栅格文件处理流程

通过配置Mapnik的样式文件,可以将透明化处理的晕染栅格文件与颜色渐变栅格文件以及原始的OSM数据层叠加,即可显示出起伏变化的立体地形地貌图效果（图5）。
Fig. 5 General view of the background tile rendering

图5 后台瓦片渲染示意图

图5中4张瓦片均表示第8层级,经度编号为210,纬度编号为96的北京同一区域。从前3张瓦片左至右分别为颜色渐变、透明化处理的晕染以及原始的OSM数据渲染瓦片,最后一张瓦片由前3张瓦片叠加而成,也即瓦片服务后台最终渲染的效果。
在较高的缩放层级下,90 m SRTM高程数据已经不能满足其精度要求,因此本文中在第0-13个缩放层级中显示地形图背景,而在第14-18个层级只显示原始的OSM数据。最终导航电子地图的渲染流程如图6所示。
Fig. 6 Rendering process of the tile server background

图6 瓦片服务后台的渲染流程

3.4 地面站软件中地图客户端模块的实现

地图客户端模块需要具备如下3个功能。首先,能够根据地图参数,使用HTTP协议根据瓦片列表从后台获取瓦片并显示成电子地图;其次,可以在电子地图上绘制航迹、飞行器当前的位置等状态信息;最后,能够对用户的操作或接收的数据进行响应,并对地图进行更新。其中从后台获取瓦片并进行绘制是地图客户端模块的关键。
文献[14]给出了特定缩放级别下经纬度和瓦片标号的换算公式[14]。本文中需要使用的变量如表1所示。本文显示地图的面板大小不变,面板的中心设定为无人驾驶装备回传的经纬度坐标。瓦片绘制的示意图如图7所示。其中深灰色区域表示显示面板,中心点用红色圆圈表示;粗虚线围成的浅灰色区域表示瓦片序列,由虚线围成的正方形表示每张瓦片,中间的浅蓝色瓦片即为中心点所在的瓦片。在一次瓦片获取过程中,首先根据显示面板中心点所表示经纬度坐标计算出所在瓦片的经纬度标号cen_x和cen_y,以及偏移off_x和off_y[14]。然后根据显示面板的宽度和高度可以确定瓦片序列的经纬度标号范围（即left_x、right_x、up_y、down_y）以及每个瓦片在地图面板中的显示位置。
Tab. 1 Variables used in the map client module

表1 地图客户端模块中所使用的变量

变量 描述
lon 地图中心点的经度
lat 地图中心点的纬度
z 当前缩放层级
fx 地图中心点所在瓦片经度标号计算值
fy 地图中心点所在瓦片纬度标号计算值
cen_x 地图中心点所在瓦片的经度标号
cen_y 地图中心点所在瓦片的纬度标号
off_x 地图中心点经度坐标在所在瓦片的偏移
off_y 地图中心点纬度坐标在所在瓦片的偏移
w 瓦片的宽度,即256
h 瓦片的高度,即256
pw 窗口的宽度
ph 窗口的高度
left_x 地图面板最左侧瓦片的经度标号
right_x 地图面板最右侧瓦片的经度标号
up_y 地图面板最上方瓦片的经度标号
down_y 地图面板最下方瓦片的经度标号
Fig. 7 General view of the tile plotting in the map client module

图7 地图客户端瓦片绘制示意图

瓦片经纬度标号范围的计算如式（1）-（4）所示。
left _ x = ( 1 2 × pw - off _ x ) / w （1）
rig h t _ x = ( 1 2 × pw - ( w - off _ x ) ) / w （2）
up _ y = ( 1 2 × p h - off _ y ) / h （3）
down _ y = ( 1 2 × p h - ( h - off _ y ) ) / h （4）
通过地图中心所在瓦片的偏移可以确定所有瓦片的显示位置。假设某张瓦片的经纬度标号为x和y,那么该瓦片左上角顶点在地图面板的显示坐标（coord_x, coord_y）,如式（5）和（6）所示。
coord _ x = 1 2 × pw - off _ x + x - cen _ x × w （5）
coord _ y = 1 2 × p h - off _ y + y - cen _ y × h （6）
在地图面板上绘制完所有的瓦片之后,还需绘制飞行航迹,当前的时间、经度和纬度等飞行状态数据,从而形成一个完整的绘制周期,具体流程如图8所示。
Fig. 8 A plotting period of the map client module

图8 地图客户端模块绘制周期

4 应用实验与结果分析

本文使用可以覆盖全中国的原始OSM数据,大小为249 M;能够覆盖中国的90 m SRTM数据,共1899个文件,大小为5.1 G左右。对90 m SRTM数据进行3种采样,将其分辨率处理为5760、720和90 m,并分别在0-4、5-9以及10-13的缩放层级下进行显示。表2是最终处理的栅格文件信息,“C”表示颜色渐变栅格文件,“S”表示地形阴影文件。如表2所示,栅格文件所占的空间远远大于同样的地域范围下矢量的原始OSM数据。
Tab. 2 Information of the raster files after processing

表2 处理后的栅格文件信息

5760 m C 5760 m S 720 m C 720 m S 90 m C 90 m S
文件数量/个 2 2 63 63 3300 3300
大小 3.3 M 4.4 M 221.2 M 295.5 M 13.2 G 2.94 G
将地面站软件所在的主机和瓦片后台主机部署在同一网段,地图客户端模块即可通过HTTP请求获取瓦片,并显示导航地图。本文中,部署了两台地面站软件和一台瓦片服务后台主机,网络配置如图9所示。
Fig. 9 Ground station software requesting tile services from the backstage supporter

图9 地面站软件向后台请求瓦片服务

本文实现了无人机地面站并成功用于无人机动力学仿真系统,如图10所示。在该仿真系统中,动力学模型软件可以对地面站软件给出控制指令,并将仿真的状态数据返回地面站,地面站数据可以根据状态数据实时更新地图,并绘制航迹以及其他状态信息。
Fig. 10 A simulation system of the UAV flight dynamics

图10 某型无人机动力学仿真系统

图11是地面站软件中导航地图的显示效果,其中图11（a）为第4缩放层级下未收到任何动力学仿真数据时的地图,图11（b）为第9缩放层级下完成某次航迹装订仿真任务时的地图,绿色线表示本次任务过程中根据接收到的9567个航路点绘制的航迹。
Fig. 11 The display effect of a map panel at different zoom levels

图11 不同缩放层级下的导航地图显示效果

5 结论

OpenStreetMap作为一种完全免费开放并可自由编辑的地图服务,在国内外被广泛应用于科学研究和项目开发。本文使用原始OSM以及90 m SRTM数据搭建了能够立体显示地形地貌的瓦片服务后台。通过实现地图客户端模块,成功将瓦片服务后台应用到无人驾驶装备的地面站导航电子地图中。该方法可不依赖网络使利用规模适中的数据显示范围广、内容详细的导航地图成为可能。使用本文中显示高程数据的方法,也可以将栅格格式的植被、人口等数据显示在地图中,从而使应用于无人装备的地面站导航地图能够提供更多类型的信息。
本文提出的导航电子地图设计方法还有如下不足：①相比于美国、日本以及大部分欧洲国家,中国部分的原始OSM数据量并不大,数据的完善程度和更新速度还有一定的差距,对精度要求较高的任务场景可能需要多源数据进行验证。② 本文所采用的90 m SRTM在缩放层级较低或适中的情况下尚可通过不同采样进行快速显示,而较高缩放层级下精度已不能满足要求,这极大地限制了地面站软件所能提供的高程支持。

The authors have declared that no competing interests exist.

References

Publishing order | Descend order by publishing year | Descend order by cited within
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DOI

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DOI

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DOI

[19]
Amir P, Jeremy M, Steven F, et al.Towards an authoritative OpenStreetMap: Conflating OSM and OS opendata national maps’ road network[J]. ISPRS International Journal of Geo-Information, 2013,2(3):704-728.The quality aspects of OpenStreetMap (OSM), as the global representation of crowd-sourced mapping, have always been of priomary concern to academics. While the methodologies for checking its quality against the national maps have been implemented by a number of studies, there are minimal works on how to practically improve the quality of OSM towards being an authoritative map source. This paper presents a method for conflating road attributes, namely the name and reference code, of OSM with the Open Data provided by Ordnance Survey (the British national mapping agency). The added values in the proposed methodology include the daily updates and serving of the conflated maps via open Web Services. More importantly, the OSM crowd correction is facilitated by frequently highlighting and web-serving the individual differences. There are currently over 5,800 differences in matching road names and references between the two datasets. In addition to describing the conflation methodology, the different geographic distribution patterns of the identified differences are discussed. A negative effect of the road density on the ratio of the mismatched features between the two datasets is observable, evidenced by their different geographical distribution over the map. It is shown that the best correspondence between attributes exists in the very dense areas, followed by the very low density areas, and lastly in the middle to large sized cities.

DOI

[20]
Kennelly P J.Terrain maps displaying hill-shading with curvature[J]. Geomorphology, 2008,102:567-577.Many types of maps can be created by neighborhood operations on a continuous surface such as provided by a digital elevation model. These most commonly include first derivatives slope or aspect, and second derivatives planimetric or profile curvature. Such variables are often used in geomorphic analyses of terrain. First derivatives also provide subtle enhancements to hill-shaded maps. For example, some maps combine oblique and vertical illumination, with the latter reflecting variations in slope. This study illustrates how second derivative maps, in conjunction with hill-shading, can cartographically enhance topographic detail. A simple conic model indicates that image-tone edges where slope or aspect varies by less than 0.5 are visible on curvature maps. Hill-shaded images combined with curvature enhance the continuity of naturally occurring tonal edges, especially in strongly illuminated areas. Variations in planimetric and profile curvature seem to be especially effective at highlighting convergent and divergent drainages and variations in erosion rate between or within sedimentary units, respectively. Shading curvature with consideration given to illumination models can add detail to hill-shaded terrain maps in a manner similar to cognitive models employed by map viewers.

DOI

[21]
韩李涛,范克楠.三维地形颜色渐变渲染的光滑过渡方法研究[J].地球信息科学学报,2015,17(1):31-36.地形颜色渐变渲染是表达地形起伏变化或其他地学要素空间分布变化趋势的常用方法。在三维地学交互分析系统中观察分析三维地学信息时，需要依据观察位置和分析区域的范围大小不停地变化视角和视距。当观察距离很近时，利用包含一定颜色数的色带渲染地形会出现明显的颜色分层现象，不能很好地表达地形颜色的光滑过渡。针对三维地学交互分析系统对地形颜色渐变渲染的操作需求和存在的问题，分析了RGB和HSL两种颜色模型的特点，顾及OpenGL的颜色平滑过渡原理，在RGB颜色空间通过对所有颜色分量进行线性插值实现了地形双色渐变渲染，在HSL颜色空间通过固定饱和度、亮度，对色相进行线性插值实现了地形多色渐变渲染，并通过加入光照计算来增强地形颜色渐变晕渲的三维立体效果。实验结果表明：本文方法能很好地兼容OpenGL的平滑着色，实现三维交互环境下任意距离观察地形均能保持颜色之间光滑渐进过渡，达到更为光滑的地形颜色渐变渲染效果。

DOI

[ Han L T, Fan K N.Research on smooth transition of color rendering for 3D terrain[J]. Journal of Geo-information Science, 2015,17(1):31-36. ]

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