A Strategy for Loading Oblique Photogrammetry Models and Multilayer Basemap Data

With the development of drone technology and oblique photogrammetry technology, the acquisition of oblique photogrammetry models and basemap becomes more and more convenient and quickly. The increase in the number of basemap leads to excessively redundant basemap tiles requests in 3D GIS when loading oblique photogrammetry models, which slows down the system. Aiming at improving the speed of running system, this paper proposes a dynamic strategy for loading basemap tiles. Different from existing 3D GIS which loading oblique photogrammetry models and basemap tiles independently, this strategy dynamically loads basemap tiles depending on different height of view and the range of loaded oblique photogrammetry models. We achieve dynamic loading of basemap tiles by predetermining whether the basemap tiles will be covered by the oblique photogrammetry models. The experimental results show that this strategy can greatly reduce the number of redundant requests from the client to the server while ensuring the user’s visual requirements for the oblique photogrammetric model.

, the application of 3D GIS gradually changes from PC-end to browser. Cesium [3] is a webgl-based [4] 3D GIS browser-side open-source framework written by JavaScript, which can be applied without any plug-ins. Following the breakthroughs in drone technology and oblique photogrammetry technology, massive data has been injected into 3D GIS.
Recently, there is no personal computer has the ability to satisfy the requirementsof loading the basemap tiles and oblique photogrammetry models at a time. Besides, 3D GIS not only need to load oblique photogrammetry models to display the realistic effects, but also need to load the basemap tiles with different functions. However, as the number of the basemap tiles increase, the number of requests sent by the client to server increases, resulting in excessive load on server. Eventually, the speed of the 3D GIS will drop off. This paper proposes a dynamic strategy for loading basemap tiles to solve the above problem. The main idea of the strategy is that the basemap tiles loading is determined by the height of view and the range of the oblique photogrammetry models.

Data Acquirement
We have acquired the raw images by drone equipped with sensors in five directions: vertical, font, rear, left and right [5]. Approximately geographical coordinates in each image were recorded during aerial photography. Meanwhile, the ground surveyors provide ground control points with more accurate geographical coordinates. Then, these raw images with geographical coordinates can be converted to oblique photo-grammetry model by Context Capture Master. Finally, in order to obtain the final data in b3dm format, a series of format conversions have been applied.

Data Storage
As for the storage of oblique photogrammetry models, this paper adopts HLOD data structure [6] [7]. As Figure 1 shown, from top to bottom, data resolution becomes higher and higher. Regular grid partitioning will be applied to single file which is extremely large. The files in different levels are named by the method shown in Figure 2. Figure 2 showed that data in different levels has different filename length. And the data index of the same level is different. Not only the hierarchical structure but also the geographical extent is recorded by the in-

Strategy Data Loading in Cesium
Cesium adopts the OOC (Out of Core) algorithm [9] for loading massive data.
Due to the limited computer memory, the global data loading and processing at once are impossible. Therefore, only the data within the view frustum will be loaded and processed [10] [11].
The loading strategy of basemap tiles and oblique photogrammetry models in Cesium is: the loading of the basemap tiles and the oblique photogrammetry models are decided by the height of view and the scene range. Once the height of view is high and range of scene is large, Cesium loads low resolution basemap tiles only. On the contrary, once the height of view is low and range of scene is small, Cesium loads relatively high resolution basemap tiles and oblique photogrammetry models. Another case is that Cesium will load higher resolution basemap tiles and lower resolution oblique photogrammetry models when the height of view and range of scene are moderate.
However, the whole scene will be covered by the oblique photogrammetry models when the height of view is low and the scene range is small. At the moment higher resolution basemap tiles in covered area don't need to be loaded.

Basemap Tiles Dynamic Loading Strategy
When the height of view is low, both the oblique photogrammetry models and basemap tiles require to be requested, loaded and rendered. Nevertheless, if the number of basemap layers is too large, the requests sent by the client increase, the system runs slower. Therefore, the excessive load of server can be relieved by reducing the requests of the basemap tiles from client. When the requests from client decrease, performance of 3D GIS will improve greatly.

Coverage Area of Oblique Photogrammetry Models
Oblique photogrammetry models and basemap tiles exist in the form of tiles.
The range of oblique photogrammetry models is determined by the way of data division. While the range of basemap tiles is determined by specific data partitioning standard. The data division criterion between the oblique photogrammetry models and basemap tiles are different.
In order to speed up scene loading, the covered range of oblique photogrammetry models should be calculated in advance. The geographical range of each oblique photogrammetry model is recorded in the index file. Therefore, covered range in certain area can be calculated by the union of all oblique photogrammetry tiles (Some areas are shown in Figure 5).

Restriction Requests to Basemap Tiles
The strategy for dynamically loading basemap tiles means that the loading of

Experiment
In order to demonstrate the performance of our strategy. In this section, the ex-

Conclusions
In 3D GIS, loading multilayer basemap and oblique photogrammetry models simultaneously will lead to excessive number of requests from the client, which slows down the speed of 3D GIS. This paper proposes a strategy for basemap tiles dynamical loading. The strategy loads oblique photogrammetry models preferentially, and limits the requests of basemap tiles. In other word, the loading of basemap tiles is affected by the oblique photogrammetry models during the browsing from the users. It is not necessary to load the basemap tiles if they are covered by oblique photogrammetry models. The experiment shows reduction in the number of requests for basemap tiles can greatly improve the performance of 3D GIS.
The requesting, loading and rendering of b3dm oblique photogrammetry models will not be affected by restricting the requests of basemap tiles. Consequently, the strategy proposed by this paper can improve the running efficiency