Keywords: Digital Orthophoto Map (DOM) Production

Abstract
To meet the needs of the flood prevention in the seven biggest river valleys of China, China National Bureau of Surveying & Mapping produces more than 11,000 sheets 1-meter resolution digital orthophoto map (DOM) in 1998 and 1999. This article introduces the progress of the digital orthophoto map production in China. Considering the 1-meter resolution digital orthophoto map production project in the seven biggest river valleys of China, it detailed describes the technical normalization, technological process, technical methods, quality control, data management and DOM database construction of digital orthophoto map production.
It also gives some key applications of the digital orthophoto map.

Introduction
Digital Orthophoto Map (DOM) is a kind of image maps that have geometry precision of map and characteristic of image. It has lots of merits such as high precision, abundant information, simple data structure, quick production and short update period etc. In the National Spatial Data Structure, DOM is the basic data layer. Furthermore, in the strategic plan of "Digital Earth" and "Digital China", high resolution DOM, that covers all the country even all of the global, is also the basic data layer in the Digital Global Spatial Frame in the future. The DOM will become a united carrier of Society, Economy and Humanity.

Recently, the 4D survey product has been listed as a part of the basic survey production plan by NBSM. The so called "4D" is Digital Raster Graphic (DRG), Digital Orthophoto Map (DOM), Digital Elevation Model (DEM) and Digital Line Graphic (DLG). Furthermore, using all kinds of the possible aerial photos and satellite images, China plan to arrange the production of 1-meter, 5-meter and 10-meter resolution DOM which cover all the country in 3-5 years.

From October 1998 to April 1999, more than 11,000 sheets 1-meter resolution DOM have been made in China. This production, arranged by NBSM, includes 7 biggest river valleys.

In addition, in recent years, NBSM have arranged to make 5-meter and 10-meter resolution DOM, using aerial photos and satellite image data, then update the corresponding DRG.

The DOM production in China is introduced in detail in the following paragraph.

1-meter resolution DOM production in 7 biggest river valleys

Task Summarize
To meet the needs of the flood prevention and reconstruction, NBSM arranges to make the 1-meter resolution DOM of the 7 biggest river valleys(the Yangzi River, Huanghe River, Zhujiang River, Huaihe River, Haihe River, Liaohe River, Nenjiang River and Songhuajiang River), which cover the following 19 provinces, autonomous regions or municipalities: Gudong Province, Hunan Province, Hubei Province, Jiangxi Province, Zhejiang Province, Shanghai City, Jiangsu Province, Anhui Province, Henan Province, Hebei Province, Shandong Province, Shanxi Province, Tianjin, Beijing, Liaoning Province, Chongqing, Heilongjiang Province and Neimonggu Autonomous Region. Ten units of surveying and mapping take part in this project. There are Shanxi Bureau of Surveying and Mapping (BSM), Heilongjiang BSM, Sichuan BSM, Hainan BSM, Shangdong Provincial BSM, Guangdong Provincial BSM, Henan Provincial BSM, Jiangsu Provincial BSM, Tianjin Academy of Surveying and Mapping and Chinese Academy of Surveying and Mapping (CASM). The Total Number of the DOM products is more than 11,000 sheets.

Product Files
The products include DOM data files, DOM cartographic files and metadata files. In which files, the DOM data files, by reason of the difference of photogrammetric materials, are divided into two types: 8 bit grey image data and 24 bit color image data, are stored in RAW format. The corresponding DOM cartographic files are stored in CorelDraw CDR format. The thematic information and annotations are saved as vectors in different layers in the CDR file.

Technical Level
The DOM image ground resolution is 1 meter.

Compared with the 1m resolution topographic the standard errors of the coordinate of the objects in DOM are not exceed what the following table shows.

Unit: mm

Terrain Type	RMS of Points	Tolerant Error of Adjoining DOM
Flat and hills	0.75	0.8
Mountain	1.1	1.2 (individual 1.6)

For the special difficult fields, the standard error of the points can be raised to more than a half. The biggest errors are defined as the doubled standard error.

Technological Flow and Methods
Shown as the figure 1, the Single Image Digital Differential Rectification (SIDDR) method and the Digital Photogrammetry (DP) method are used in this production. The left part of the dash line is the DP method, the right is the SPDDR method.

Control Schemes
According to the difference of control materials, there are several control schemes in use: use the existing result material of ground densification points, read coordinates and re-densification in 1-meter topographic map, make DRG or use existing DRG to get the coordinates of control points.

Figure 1. The technological flow and methods in 1-m resolution DOM production.
Methods of Obtain DEM
Some working fields have DEM in which the grid interval is from 12.5 to 25 meter. Some fields have no DEM data but have topographic map. In these fields we can obtain DEM data by means of digitize the topographic map and extract the information of Elevation.

In the field that have no DEM and have no topographic map, man can get DEM data by photogrammetric means.

Methods of Ortho Rectification
There are two main methods in Ortho Rectification for aerial photos. One is the Single Image Digital Differential Rectification method, the other is the Digital Photogrammetry method. The Former is often used in production because its have lots of merits, such as man can directly use the existing DEM data to rectify and can use parts of photos to rectify and this method is fit for organizing large scale production. The latter is often used in the fields that have neither DEM data nor topographic map.

Overlapping of Thematic Information
DOM planning map is made by overlap the annotation information and other thematic information on DOM.

The thematic information include:

Name annotation: such as place name, factory and mine name, road name, water name, dyke and important dam name etc. Administrative boundaries: such as country boundaries, province boundaries, city boundaries and county boundaries etc.

Main roads: such as railways, highways, national roads, provincial roads and common roads etc.

Elevation information: such as the contour near the highest flood level, the elevation point near and below the highest flood level.