六百年前鄭和下西洋，帶領龐大船隊從南京龍江關出發抵達波斯灣的忽魯謨斯。途經數十個國家，除了要靠沿岸陸標導航外，在橫跨浩茫險阻的大海時，尚要靠牽星來過洋。
本文藉助《武備志》《自寶船廠開船從龍江關出水直抵外國諸番圖》及其中所附四幅《過洋牽星圖》，作為深入探討的主要根據資料。前者記載從南京到忽魯謨斯沿途航線上的山川、地勢、島嶼及針向；後者則記載船隊從蘇門答剌至忽魯謨斯間，橫渡孟加拉灣及阿拉伯海時，所倚靠各方星辰的導航圖。這兩類航海圖，顯示鄭和當年是參考陸標與天文兩種方式來完成這項遠程的航海任務。
本研究欲解決的問題有三：1. 一指仰角到底等於幾度？2. 為何北辰星與燈籠星是搭對出現的？3. 長期未能定位的丁得把昔及沙姑馬山，其位置究竟為何？
根據《順風相送》所載古里北辰星仰角為4 指，及其真確緯度為11.3 ºN 兩項因素，可以推算得到一指仰角等於1.57 º的換算值。又因明初北辰星的去極度甚大，他們定位的法則是當北辰星與燈籠星兩者仰角和為15.5 指時，則北辰星指數可以作為緯度高低的指標。利用上述兩項結果，丁得把昔及沙姑馬山兩個長期以來未能定位的地點，可以認定他們的緯度分別為16.4 ºN 及22.6 ºN。

Zheng-he sailed across the Gulf of Bengal and the Arabian Sea many times in the early 15th century. Each expedition involved thousands of crewmen, hundreds of ships and visits to tens of countries. The fleet took off from Nanjing to Hormuz by means of terrestrial navigation and astronomical navigation.
Official records about Zheng-he’s expedition are sparse and meager. These
valuable documents reveal the secrets of ancient navigation techniques and include two types of sea charts as appeared in Wu-Bei-Zhih. The first type, called Land Mark Navigation Chart includes 40 consecutive maps that sketch out land marks such as mountains, rivers, islands, buildings and temples along the sea routes. These maps also recorded the compass direction and the depth of the sea floor on the routes from Nanjing to Hormuz. As such, these maps are regarded as terrestrial navigation maps.
While the second type, called Star Map, recorded the star elevation angles as observed at Calicut, Tondavali (Dandi Baxi), Sha-Gu-Ma-San (Mountain Sham), and are regarded as astronomical navigation maps. Major stars used for positioning the ships include Polaris, Gacrux, Vega, Pollux, and Procyon.
Research results are summarized as follows: 1. Using Calicut’s latitudinal
coordinate and the star observation records, one ‘finger’ is figured out to be 1.57º. 2. Back in 1433, when the sum of the elevation angles of Polaris and Gacrux add up to 15.5 ‘fingers’, then navigators recorded the Polaris elevation to indicate the latitudinal position of a place. 3. Two unknown places such as Dandi Baxi and Sa-Gu-Ma-San, could now be positioned at 16.4 ºN and 22.6 ºN respectively.