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Objectives: The pollution of heavy metal has become increasingly serious in recent years. The accumulation of heavy metals in the soil will be a threat to ecological balance and human health. Therefore, we need to obtain heavy metal content in soil quickly and accurately. Methods: This paper proposes a method to combine field and laboratory spectra to construct a mechanism estimation model of soil lead (Pb). Firstly, direct standardization (DS) algorithm was employed to eliminate the influence of environmental factors on the field spectra. Secondly, in order to enhance the diversity of the samples, the laboratory spectra were introduced to joint modeling. Finally, the characteristic spectra of iron oxide were extracted for modeling to increase the model rationality. Results: This method was validated by the spectra of 70 soil samples from Xiong'an farming area in Hebei province. The accuracy R² of model established by full-band field spectra without DS correction was only 0.220 0. However, the accuracy R² of model established by the pro⁃ posed method in this paper reached 0.914 6.Conclusions: It indicates that the model for estimating Pb content can be significantly improved by removing the influence of environmental factors on the field spectra, extracting the iron oxide characteristic spectra of the combining field spectra with laboratory spectra. [ABSTRACT FROM AUTHOR]

Objectives: Slow convergence has always been an important factor of limiting the development of precise point positioning (PPP). Studies have shown that the convergence speed of PPP can be significantly improved with high ‑precision ionospheric delay correction, and then to achieve PPP ‑ real time kinematic (PPP ‑RTK). At present, the ionosphere in the regional PPP ‑RTK mainly adopts the satellite ‑based ionospheric model with polynomial function (SIM_POLY) and the satellite‑based ionospheric model with in‑ verse distance weight function (SIM_IDW) for construction. Methods: In order to verify the modeling accuracy of the above two models at different latitudes, this paper first used the observation data of Guangdong, Hubei and Hebei provinces to establish ionospheric delay models, and then applied the above models to float and fixed solutions PPP under single GPS and GPS+BDS systems. Finally the results were compared with IFPPP (ionosphere‑free PPP) and CODE GIM (Centre for Orbit Determination in Europe Global Ionospheric Map, CODG) correction. Results: Experimental results showed that in the low‑latitude provinces, SIM_IDW model slightly outperformed SIM_POLY model, but there was no significant difference in the middle and high latitude provinces. Compared with IFPPP and CODG correction, PPP under SIM_IDW and SIM_POLY correction has a better performance. Besides, Hebei province achieved the fastest convergence speed compared with Guangdong and Hubei provinces, and compared with IFPPP, the positioning accuracy in single GPS solution under SIM_IDW and SIM_POLY model correction is improved by 43.7% and 43.0%, respectively. In the fixed PPP, the success rate of first epoch to fix the ambiguity of PPP‑RTK under the correction of SIM_IDW and SIM_POLY models in GPS+BDS solution could reach 86.09% and 89.13% in Hebei province. Besides, positioning accuracy could converge to 5 cm in the first epoch in horizontal direction, and converge to 10 m within 1.5 minutes in vertical direction. The positioning accuracy after convergence of GPS+BDS PPP ‑RTK had a significant improvement compared with single GPS, which was 1.3 cm in horizontal direction and 3.5 cm in three‑dimensional direction under the correction of SIM_IDW and SIM_POLY models. Conclusions: Establishing a regional ionospheric model through SIM_IDW and SIM_POLY models to realize PPP‑RTK can significantly shorten the PPP convergence time and improve positioning accuracy. [ABSTRACT FROM AUTHOR]