Metadata files
For the demands of database construction and management, all of the DOM planning map has a corresponding metadata file. There are 71 items in each file. The items include the information of reference symbols, coordinate of the four corner, projection, reference global, reference coordinate system, reference elevation system, aerial photography, obtaining data, recording of quality, distribution condition etc.

Working Soft System
Three soft systems had put into used in this production. There are PhotoMapper DOM Making System, VirtuoZo Digital Photogrammetric System and JX-4A Digital Photogrammetric System.

The PhotoMapper system, developed by CASM, is a professional soft system to product DOM. Using it man can quickly rectify aerial photos and satellite images to DOM. Most area’s products are made by the PhotoMapper system in this production. The VirtuoZo digital photogrammetric system is developed by Wuhan Technical University of Surveying and Mapping. JX-4A digital photogrammetric system is developed by CASM.

1:50,000 DOM Production Plan

Background
Campare with other scale DOM, 1:50,000 DOM is used the most widely, but we have fewer 1:50,000 DOM now. In order to solve the problem of updating Chinese 1:50,000 scale basic topographic map (DRG and DLG), and meet the needs of national basic surveying and mapping, take account of the investigation of land resource in all the country, NBSM establish a 1:50,000 DOM production plan. According to the plan, the 1:50,000 DOM production will be accomplished in 3-5 years. The resolution of DOM is 1 or 5 meter (aerial photos as source data) and 10 meter or 15 meter (satellite images as source data).

Product Types
For the reason of the source of image data are aerial photos (B/W or Color) and satellite images (panchromatic or multi-spectral images) in 1:50,000 DOM production, the types of product include: 8- bit grey DOM, 24-bit color DOM and multi-spectral DOM (8-bit in each band).

Technological Flow and Methods
For the difference of the source data reason, the 1:50,000 DOM production is divided into two ways: the technological way based on aerial photos and the way based on satellite images. The former, which likes the technological way of 1-meter DOM production, is also divided into the Digital Differential Rectification (SIDDR) method and the Digital Photogrammetry (DP) method.

Different to the way based on aerial photos, there are several differences in the latter way.

Digital Geometric Rectification
The method of geometric rectification includes polynomial geometric rectification and digital differential rectification. The method of polynomial rectification suits for the flat areas, and the digital differential rectification suits for the hills and mountains or the case of SPOT data with big inclination angle.

Image Merging
In order to make the best of the information of multi-spatial RS data, the merging technique is often used in 1:50,000 DOM production.

The following methods of image merging often in use: IHS color transformation, principal components transformation, algebraical method and wavelet transformation.

The Application of DOM in China

DOM is national high precision space basic data
As one of the national basic data surveying and mapping products, DOM is the basic data of National Space Data Instrument (NSDI) and the Global Space Data Frame in the strategy of digital earth.

To a great extent, DOM is the base of the future social management, emulational function analysis operation, and the suppositional operation analysis of the Digital Earth in the future.

DOM is the main data to update the national basic topographic maps
DOM is a kind of image map that reflects the real conditions on earth's surface and has a strong present-time characteristic and geometrical precision. It will be considered as the basic data source to update and revise the future national basic topographic maps (DRG and DLG). Using DOM to update the topographic maps will be a main technical method for China to update the future relief maps. Our present main task is to update 1:50,000 topographic maps.

DOM is the basic data of different special field
DOM has such characters as multi-phase, multi-band, high resolution, short production periods, strong present-time character, abundant information and so on. It can do as the basic data or basic map in all kinds of scientific fields. It offers all the fields a united high precision space positioning frame and a solid base on which we can achieve the plans of National Spatial Data Instrument (NSDI) and the Digital Earth.

DOM is used in the following fields:

• The detailed investigation of the land resource and its changing
• The dynamic monitoring of the land use
• The investigation of forest resource
• The investigation of agriculture resource-- the monitoring of planting acreage of the crop and the product estimation of the crop
• The flood prevention application
• The monitoring of environment
• The planning of urban
• National large basic instrument construct project

The Compound Application of 4D products
The application of 4D products is more and more developed though compound DOM and the other digital surveying and mapping products (DEM, DLG, DRG) and the data of GIS. For example:

Using compound of DOM and DEM, man can make 3-D static natural landscape map. The map can be used in lots of fields, such as tourism, planning, forestry, environment, land-use etc.

By means of the technology of 3-D visualization, using the data of DOM and DEM, man can realize the 3-D dynamic real simulation. This technology can apply in military and large water conservancy project.

Conclusions
DOM production will be used widely in China as a new image-map production in the future, for its high resolution, rich information, and also for its shorter period of producing and renewing. But several problems will be considered in the large scale DOM production.

• How to integrate DOM images with different resolution, coordinate system into a united database is still needed study.
• It would be useful to build the control point database of the whole country to guarantee DOM with the unite coordinate system and higher precision.